THE INTERORBJT-4L SEYTUM I N MAMMALS
’
453%
The Interorbital Septum in Mammals. By R.WHEEL= HAINES,
DSc., Department
of Anatomy, Faculty of Medicine, Abbassia, Cairo.
(With 40 f l g u m s in the text.)
INTRODUCTION.
I n most living reptiles a thin sheet of cartilage, which may be calcified,s e p a r a h
the orbits from each other, and in birds a sheet of bone occupies the same position
and is a conspicuousfeature of the skull (Bellairs, 1949). Inmammals (Gaupp, 1900)
~ ~ o @ ; n i z the
e d region of the central stem of the chondrocranium that lay between
the nasal capsules anteriorly and the pituitary region posteriorly as the morphological
equivalent of the reptilian interorbital septum, but he did not realize that typical
well-developed septa could be found in mammals. Fischer (1903), however, found
such a structure in a late embryo of an Old-World monkey, Semmpithcus, and figured
his discovery side by side with a figure of the lizard h r t a from Gaupp’s monograph
t o demonstrate the morphological identity of the septa.
Voit (1909) found the septum in the late embryo of the rabbit, and other workers
have described it in a number of different animals. de Beer and Woodger (1930)
studied its early development in the rabbit, and reviewed the work of previo~s
authors. They classified this material according to the strong, feeble or nondevelopment of the septum, and concluded (p. 403) that the only mammals in which
it was well developed were the primates and rodents, and this conclusion is repeated
in de Beer’s book on the skull (1937). I n the present paper the distribution of the
septum will be reconsidered, and it will be suggested that m regards this feature
the ungulates have as good a claim as the rodents to be c l a ~ e dwith the primates.
The significant gaps in our knowledge relate, however, not to the chondrocranium
. of the embryo but to the skull of the adult. The monographs which deal in detail
with particular types hardly mention the matter, and Muller (1925), who wrote
on the orbito-temporal region of the adult skull, knew the typical septum only in
primates and rodents. The well-developed septa of the smaller deer and antelopes
and of some carnivores seem to have escaped the notice of morphologists altogether,
those of the eared seals have been misunderstood, and illustrations designed to show
the structure of the septa are not available anywhere in the literature.
The purpose of the present paper is to describe and illustrate the position and
structure of the adult septum in all groups of mammals known to possess one, to
compare its adult form with that in the chondrocranium where the latter has been
studied, and to consider its morphological and phylogenetic significance*.
LAGOMORPHA
AND RODENTS.
The rabbit, Oryctohgus cuniculus, has a small but typical interorbital septum,
and since it is a readily available form whose structure and development have been
. exhaustively studied, this animal may be chosen to illustrate the subject. A side
view of the skull (fig. 1) shows the orbit bounded above by the frontal, anteriorly
by the la&rymal and maxilla, below by the zygomatic (fused with the maxilla),
and the zygomatic process of the squamosal, while posteriorly the orbit is confluent,
with the temporal fossa. The bones mentioned build the interior walls of the orbit
with the help of the palatine and alisphenoid below, and in the deepest part of the
orbit there is a wide exposure of the presphenoid, with the basisphenoid just visible.
(!be term ‘ presphenoid ’ is used to denote the bone in the adult skull, separate in
most mammals, which is usually ossified from presphenoid and orbitosphenoid-centres. It corresponds to the ‘ presphenoid part ’ of the sphenoid bone of man.)
* This paper forms the second part of an investigation of the interorbital septum in andote
vertebrates originally begun in collaboration with Dr. A. d’A. Bellaira of the Dept. of Anatomy,
London Hospital Medical College. Owing to the departure of the present writer to Egypt, it
hea not been possible to continue the collaboration and the Sauropsida and Mammalia have heen
dealt with separately. Conditions in Sauropsida have been described by Bellairs (1949).
.386~
R . \\‘HEELER HAINES
:
The optic foramen lies in the presphenoid, leading from the cranial cavity @ t j q
orbit. Anteriorly the right and left foramina have a common boundary, a relatively
thin edge of bone, the posterior margin of the interorbital septum. This septum
can be demonstrated very clearly by illuminating one orbit so that the light shines
through the thin septum and can be seen from the other. In the rabbit the septum
is not very extensive and is entirely confined to the presphenoid bone.
An isolated pvespheiloid from a young animal (fig. 2) shows the two lesser wings
posteriorly, the optic foramina meeting at the interorbital septum centrally, akd the.
dnterior extensions which help to build the wall which separates the orbit from ithenasal cavity. Between these extensions the presphenoid passes a short distenwinto the nasal septum
FIG. 1.--OryctoEayzcs cziniculuu. dry skull.
FIG.2.-Oryctolayus cuniculzcs, presphenoid bone.
FIG. 3.-Oryctolaguu cuniczdus, decalcified head, horizontal section.
FIG.4.-Oryctolayue cuniculw, decalcified head, coronal section.
A decalcified head dissected from above down to the level of the optic chiasma
shows the main relationships of the presphenoid very clearly (fig. 3). The lesser.
wings separate the cranial cavity from the orbit. The portion of the cranial cavity
between the optic foramina is entirely occupied by the optic chiasma and the
terminations of the optic nerves, which here lie side by side. Followed forwards the
nerves suddenly diverge, passing through the optic foramina on either side of the
interorbital septum. The septum continues forwards to meet the anterior expanded
part of the bone which separates the nasal capsule from the orbit, and thus separates
’the optic chiasma behind from the nasal cavity in front.
A coronal (transverse) section (fig. 4) from another decalcified head shows the
septum between the orbits as a bony sheet. Below it is rounded, and the two
palatines which support the sides of the nasopharyngeal duct articulate with it.
Bbove, it carries two small wings, poorly developed here, which support the brain aqd
articulate with the frontals.
Th E ISTERORRITAL SEPTUM IN MAMJIAJ S
587
,To revert to the dry skull (fig. I ) , the outlines of the cerebral and nasal cavitiw
may be marked in on the surface of the bones. (This is accomplished by shining
light into each cavity in turn, and marking the areas of translucency on the surface,
checking the result on broken skiillsJ). It can then be seen that the interorbital septum
ia a bony partition between the orbits, bounded above by the cranial cavity, behind
by the part of that cavity that lodged the chiasma, anteriorly by the nasal cavity
with its ethniotiirbinal apparatus, and below by the naso-pharyngeal canal. Theserelationships will be found to hold generally throughout the Mammalia, but where
the chiasma lies just posterior to the nasal cavity, or the cranial cavity just above t h e
respiratory pawage, no septum is developed.
FIG. B.-Oiyctolagus cuniculus, skull base.
FIG.8.-Sciurwr carolienais, decalcified head, horizontal section.
FIG. 'I.-Sciu~us caToliensiS, skull base.
Fro. ~.--,YETUS
setomis (Camb. E. 1630), clry skull.
The floor of the cranial cavity viewed from above (fig. 5) shows the septum aa a.
relatively narrow ridge of bone between the optic foramina. A glance at a
base, or even inspection through the foramen magnum, is usually sufficient to determine the presence of a septum in any animal, for if the optic nerves split round a
narrow edge of bone that edge belongs to a septum, whereas if, as is more usual, t h e
foramina are far apart, no septum can be present.
Such condition is illustrated among rodents by the squirrel (Xciurus),for here
tqherhia,sma is X shaped, and the two optic foramina are separated by a wide interva
588
H. \VHEPI,EH H A I X E X :
(figs. 6 and 7). There ib: no trace of an interorbital septum, and the two foramina
cannot be said to have a common anterior boundary. The chiasma is 8et direotly
against the bony covering of the nasal cavity, and a hole cut in the cranial floor
between the optic foramina leads to the recess of that cavity, which contains the
fourth ethmoturbinal. These two animals, the rabbit and the squirrel, exemplify
the two common mammalian conditions in this region.
I n other rodents the septum is usually absent, but in Xerus setosus, a relative of
the squirrel, there is a septum which involves not only the presphenoid but also
the frontal, so that the suture between the two bones crosses the septum (fig. 8).
I n Xerw erythropus (Camb. E. 1528) the septum is similar, but does not involve the
frontal. In Rhinosciurwr, which like Xerw has a long narrow skull, there i R a g o d
.septum, but in most Sciilridae the skull is short and there is no septum.
kc:. !J.--l~eotln{/usp.y{/f,urf'tca(Zool. Sou.), clecbalcifieclIicsd, coronal section.
FIU. IO.--CephaZophorun coprzdew, (Camb. H. 21621), dry skull.
P'ICJ. 1 I.--Trnpdwr stadeyuncts (Camb. H. 14971), dry skull.
FIG. ld.--HPr.pe&ex yriaeua (LondonHospital Medical College), dry skull.
Among the Muridae the musquash (Ondatra z i b e t k ) has a septum which may
be perforated in the dry skull (Camb. E. 2810), or may be solid (Thos. 94), but other
genera (Mus, Neotoma, An*iwlu,H?$romys, etc.) have none, neither are septa known
in other groiips of rodents.
UNGULATES.
Septa have iiot previously been recorded in adult ungulates, but several of the
smaller species possess them. The royal antelope, Neotragzcs pygmueus, the sn.mllest,
ungulate known, has an extensive presphenoidal septum, better developed then
n any other animal oiitside the primabs (fig. 9). The eyes and orbits are, aa in
TEE 1NTERORBITAL SEP!CUM IN LMAMMALS
589,
most ungulates, large, and the size of the septum is associated with the requirements
of packing such large objects into a small skull. Similar septa are found in the related
dik diks, Moohqm saltiuna (Camb. H. 22,242), and M . phillipi (H. 22,281).
The blue buck, Cepha1ophmu-s coeruleus (fig. lo), has a septum occupying the
whole height of the presphenoid between the frontal and palatine articulations,
and it is similar in C . niger (Camb. A. 47 L) and C . hrveyi (H. 21,381). The common
duiker, C . grimmi (H. 21,796), is larger than the other members of the genus, and
here the presphenoid is reduced in height and the septum small.
In the cheaotains (Tragulus)there is a septum which is usually perforated by a
jagged fenestra in the dry skull (fig. ll),but presumably this is filled in by membrane
in the living animal. The perforation is inconstant, for in a skull of T . javanicus
(Camb. H. 15,018) there is no fenestra in the well developed septum. I n the related
musk deer, Moschus moschifem (Camb. H. 15,312), there is no septum, though the
optic foramina are still near together, so that this form provides a transition to the
usual septum-less condition of the larger deer and antelopes, and of other groups
of ungulates.
CARNIVORESAND SEALS.
In adult carnivores again the interorbital septum has not previously been recorded,
but it is found as a distinct thin bony sheet in several Mongotidm and Viverridae.
In Herpestea it is best developed in the smaller, long headed forms (fig. 12), whereas
it is poorly developed in the large, stoutly built forms such as H . bruchyurus (B.M. 21.
10.2.2). It is found in most individuals of Helogale, such as H . victorina (13.10.18.46)
and H . rufula (16.1.15.7), but may be absent in the latter species (16.1.15.9). A
broken skull of Cynictis pnicillatu (fig. 13) in the British Museum shows the relationships to the optic foramina, the cranial cavity, the posterior extension of the nasal
cavity, and the naso-pharyngeal passage. The septum occurs in other species of
Cynictis and in Surieata (2.9.1.26), and there is some indication of its presence in
Atilaz (1938.5.26.2),Eupkres (1539 a), Rhynchogale (9.11.25.1)and Dologale (9.5.12.3),
but it waa not seen in Mungos, Bdeogale, Crosearchwr, Ichneumiu etc. Among
Viverridae it was seen in Viverricuh (13.1.6.2), Prionodon (13.7.11.2),Foim (1084 a)
and adult and young &nettcG (8.6.14.7)and (4.4.9.34),but not in Fosea, Cryptoprocta,
Paradoxurw, Cynogccle etc.
I n seals Muller (1925) stated quite correctly that septa may be found in t h e
adult, but she believed these septa to be made of the palatine bones, and so to be
peculiar secondary specializations. The septa are found in the Otariidea, in several
species of &ria (fig. la), though poorly developed in 0.jubata, a very large species
(Camb. 907 N), in Arctoceplacclwr h o o k i (Camb. 904 A), and in Zalqvhus californicwr(Camb. 905A, B). They are in fact of the typical presphenoid type already
described in rodents and carnivores. The palatine bones as usual guard the nasopharyngeal passage, and articulate with the presphenoid, but take no part in t h e
formation of the septum itself. Septa are not found in typical seals (Phoca, Lobodon,
Cysto(phara) or in the walrus (Trichechwr). I n these animals the space between
the orbits is occupied, not as in typical rodents and carnivores, by the posterior
extensions of the nasal cavities, but by the nasopharyngeal duct which lies just
ventral to the cranial cavity.
~
T
E
.
S
The interorbital septum has not been found in the adult tupaiids or lemurs, but
is well developed in most typical primates, including Tarsius and New and Old World
monkeys, though it is lost in the anthropoid apes and man. In Tarsius the orbits
are enormous, and in some individuals even encroach on hhe roots of the molar teeth,
which appear as truncated stumps expossd in the orbital floor, cut off flush with the
bone (figs. 15 and 16). This feature has been Sean also in some individuals of
Herpeates. The interorbital septum in Tarsiwr is. as would be expected, very large.
,
590
R. WHEELER HAINES :
When the skull is held up to the light frontal and presphenoidal fenestrae, of about
equal size, light up, while between the illuminated areas a more opaque band follows
the fronto-presphenoid suture.
A skull broken along the horizontal plane shows the septum in section as a thin
sheet of compact bone separating the orbits (fig. 16). The septum passes from the
cranio-nasal passage anteriorly to a slightly thickened posterior margin, which
contains some cancellous bone and serves as the common anterior boundary t o the
two optic foramina. This septum is the largest known, relatively to the size of the
skull, in any animal. Woollard’s (1925)monograph agrees well with the above as
far as the individual bones are concerned but there are no detailed descriptions or
figures of the region showing the sutures.
14
1%. 13.-Cynictis penicillata (B.M. 25.1.2.83). broken skull.
E’IG. 1 4 . 4 t a r b cinerea (Camb. 911), dry skull.
FIG.lS.-!.!’arsiwr spectrum (B.M. 00.7.25.1), dry skull.
PIG.16.--Tursiu~ apectrunt (B.M. 46.1.29.2), broken skull.
.
The common marmoset, Callithrix jacchw, shows some variability in the
.constitution of the septum. The posterior part is formed of the presphenoid, and
the septum may have only a small extension onto the frontah (fig. 17),or the frontal
may take a more important part (fig. 18),or finally a small additional portion may be
formed of the ethmoid (Camb. E. 7901 E). Beattie (1927) described the presphenoidal part of the septum accurately (p. ell), but his fig. 11, of a parasagittal
section of the skull, shows a large area which must be an interorbital septum, with a
vertical suture crossing it. If the suture is correctly drawn the anterior bone must
be the frontal, not the ethmoid as it is labelled in Beattie’s figure, for the ethmoid
is not found in the antero-superior part of the septum in this or any other mammal.
His beautiful drawing (his fig. 9) of the skull base shows the optic foramina x-ery
dose together, as in the rabbit.
THE INTERORBITAL SEPTUM IN MAB1M.iLS
591
Other memberH of the genus Callithrix and other genera of marmosets agree with
the common form. A section of the skull of My8tax (fig. 19) shows a septum composed
.of presphenoid, frontal and ethmoid in that order of importance, and the septum
is seen to have its usual mammalian relationships to the cranial cavity, respiratory
apparatus and optic foramina. But since, as in most primates, ethmo-turbinals I11
and I V have been lost, it is I1 that lies nearest the margin of the septum.
The squirrel monkeys, Sairniri (Chry8othrix), have the septum replaced by a
large foramen leading directly, in the dry skull, from one orbit to the other (fig. 20).
The foramen, filled in life by a membrane, is bounded by rounded borders of the
FIG. 17.-C’uUit/wi.r j u c c h w (S.M.3.10.1.1.), dry skull.
FIG.18.-CaZZithrix jucchtw (Camb. E. 7901D). disarticulatetl skull.
FIG. 19.-.%fgataz mfiventer (B.M. 3.4.5.3), sectioned skull.
PIG. 20.-Sczimiri sciurea (B.M. 23.8.10.5), dry skull.
frontal above and the presphenoid and ethmoid below. It is mentioned by Elliott
(1914) as a generic peculiarity, and noted independently by Muller (1925), but does
not seem to have been figured hitherto. No other mammal has a foramen of this
size, but a morphological prototype may .be found in Cerwpithecus mona (fig. 31).
Septa are not developed in all New World monkeys. I n Alouattu ( M y W )
Fischer (1903) noted the absence of a septum, and the optic foramina are widely
apaced and separated by a chiasmatic sulcus (fig. 21). Now, in S c i u w , taken t~ a
typical mammal without a septum, it wae found that the optic nerves diverged round
the posterior extensions of the nasal capsules, which enclosed the ethmo-turbinals
592
H. WHEELER HAINES :
IV (fig. 6). Rut in primates these turbinah and the spaces that contained them
have been lost, so that the relationships of the chiasma must be different.
The fine skull of Alomtta from Cambridge (fig. 22) shows that the space in the
presphenoid wm taken up by an accessory air sinus. This sinus was known to
Zuckerkandl (1887, p. 66), who considered it, from its position, as a sphenoidal
sinus comparable to that of man. He noticed, however, that it could be followed
anteriorly into the ethmoidal region, and suggested that it had become secondarily
confluent with ethmoidal cells. True ethmoidal cells are now known, however, to
be confined among living primates to the African anthropoid apes and man (Cave
and Haines, 1940). and apart from pathological conditions involving breakdown
21
-v
-
V
FIG.2l.-AZouatta sp. (Camb. E. 7800 C), skull base.
FIG.22.-AZouatta sp. (Camb. E. 7600C), sectioned skull. Arrows in
maxillary sinus.
FIG. 23.--BrachyteZes hypozanthw, (B.M. 62.12.9.7). sectioned skull. Arrow in sphenoidal sinus..
FIG. 24.-Cebus sp. (Camb. E . 7670 D), dry skull.
of the sinus walls the air spaces do not communicate with each other. Further,
the space in question has no opening into the back of the nasal cavity, so that it
cannot be a sphenoidal sinus.
I n fact the space in the sphenoid opens into the side wall of the nasal cavity,
in the middle meatus, so that it turns out to be, at least in this individual, a sphenoidal
recess of the maxillary sinus, which besides occupying its ordinary position has this
posterior
extension. A precisely similar condition is found in the orang (Cave and
Haines, 1940).
A true sphenoidal sinus does occur in Brachyteles (fig. 23). Here again there is
no interorbital septum, and as in Alouattu the optic foramina are set wide apart, but
593
TEE INTERORBITAL SEPTUM IN M A M M A L S
the body of the presphenoid is occupied, apart from cancellous tissue, by a sphenoidal
sinus whose narrow opening leads directly into the nasal cavity behind the cribriform
plate. Cavities such as this must be evolved independently from those in the African
anthropoids and man, since their occurrence is sporadic in the New World monkeys
and they are not found in the Old World types, yet morphologically they are
indistinguishable from those of man.
The capuchin monkeys, Cebus, are variable in structure. In Seydel's (1891)'
C. hypkucus a pair of well developed sinuses occupied the interorbital region, and
Paulli (1900) has given an exact description of sphenoidal sinuses similar to that
shown in fig. 23. A C. macrocephalus (B.M. 1022a) confirms these descriptions
for the bone is broken away on on0 side exposing a typical sphenoidal sinus opening
25
FIQ. 26.-Macacus sp. (Thos.), dry skull.
FIQ. 26.-Macacua sp. (Thos.), section of skull.
FIU. 27.-Macacua rheaus, decalcified head, horizontal section.
FIU. 28.-Macacua rheaua, decalcified head, coronal section.
directly into the nasal cavity. But Fischer (1903, p. 401) found septa in addfi
specimens of Cebus, and fig. 24 shows a Cebus in which there is no sphenoidal sinus,.
while a typical, though small, septum is developed in the presphenoid.
The presence or absence of the septum is not associated with any particular species,
Among the skulls in the British Museum the septum may be preaent or absent in
C. albifrons (50.11.22.23 and 3.9.1.5), in C. pallidus (44.8.24.7 and 46.8.24.8), and
C. hypkucus (3.3.1.13 and '2.3.5.14),and it may be well developed in C. fatuellw
(90.2.22.3), a speciea in which Henckel (1928) failed to find it in the embryo. This
last record, though based on a single observation, has found its way into the literature
as typical of the genus.
41
JOURN LI". SOCJ..-ZOOLOQY, VOL. XLI.
594
R. WHEELER aAINES
:
In other genera examined, including Atelm, Nyctipithecus, Aotua, Pithecia, et c.
the septum was always present.
Mamcus is the best known genus of the catarrhines, and shows a typical interorbital septum, formed of the presphenoid and the frontal bones, with the frontal
component relatively large (figs. 25 and 26). A section of a decalcified head in the
plane of the chiasma (fig. 27) shows the septum formed of the single presphenoid
posteriorly, and the paired orbital plates of the frontals anteriorly, both containing
some cancellous tissue, but this is not too thick for translucency in the dry skull.
The relationships of the septum to the optic foramina and the chiasma are just as in
the rabbit (cp. fig. 3), for in, both animals the septum forms the common anterior
margin of the two optic foramina. Anteriorly the relationships are rather different,
for owing to the reduction of the nasal cavities and the fiexion of the basicranial
axis only the extreme upper parts of the cavities, empty of turbinals, appear in the
section.
--FIQ. 29.-Macacwr sp. (Thas.), skull base.
FIQ.30.-Papio sp. (Thos.), dry skull.
FIQ. 31.--Cercopithecus nwnu (Zool. SOC.), dry skull.
A coronal section through another head (fig. 28) shows the frontal part of the
septum between the cranial cavity above and the nasal cavities below. In dlacacwr,
as in most monkeys, the extent of the nasal cavities corresponds closely to that of
the ethmoid bone, which is ossified in the cartilaginous nasal capsules, so that the
lower margin of the septum follows the fronto-ethmoid suture. This is, however,
not constant, for both in New World and in Old World forms (figs. 19 and 31) the
ethmoid may enter the septum, though the frontal never encroaches on the nasal
cavities in these groups.
The skull base (fig. 29) shows the narrow interorbital septum between the optic
foramina as in the rabbit (cp. fig. 5 ) . With the reduction of the olfactory apparatus
THE INTERORBITAL SEPTUM I N U M A L S
695
the cribriform plate has been reduced to a simple cranio-nasal passage, occupying
a small field between the orbital plates of the frontal which are expanded to support
the frontal lobes of the brain. The territory between the optic foramina and the
cranio-nasal passages is formed by the bones that contribute to the septum, the
presphenoid and the conjoined frontals.
Other Old World monkeys agree with Macacw. Papio (fig. 30), for instance,
shows the septum with its characteristically large frontal component. Most species
of Cercopithecw have the same type of septum, but in C. mom (fig. 31) the ethmoid
takes part in the large septum, and between the frontal and ethmoid there is usually
a foramen connecting the two orbits, resembling that already described in Saimiri
(foramen absent in B.M. 30.11.11.60). The foramen has not been found in other
species of the genus with the exception of C. grayi (B.M. 0.2.5.4),nor in other genera
.of Old World monkeys. The skull base of C . aethiop, figured by Wood Jones (1929)
,resembles that of Macacus, but the interfrontal articulation is more elongated.
The septum itself may occasionally be absent as an individual variation, for Fischer
(1903) found it so in a Macacus, and it is also absent in Nasalis hrvatus (Camb. E.
7821 A and C). Unfortunately no cut or broken skull of this species has come to
hand, so it has been impossible to discover whether an accessory air sinus has inflated
the presphenoid.
There are no interorbital septa In the anthropoid apes, for the sphenoidal body
is always occupied by an air sinus. This sinus, just as in the New World monkeys,
may be of two kinds, a true sphenoidal sinus as in the gibbons, chimpanzee and
gorilla, or an extension of the maxillary sinus as in the orang (Cave and Haines, 1940).
In man the sinus is usually the sphenoidal, but occasionally a posterior ethmoidal
,cell may invade the sphenoid and take its place to a greater or lesser degree.
FI G . 32.-Macropus gigalzteus (Bulgekow), dry skull.
FIG.33.-Macropus gigantewr (Bulgakow), skull base.
GROUPSWHERE INTERORBITAL
SEPTUM
IS ABSENT
IN THE ADULT.
The interorbital septum has not been found in the adult in monotremes, marsupials, edentates, insectivores, bats, &leopithecua, tupaioids or lemurs. I n N a c ~ o p a ,
for example (fig. 32), the ‘ posterior root ’ of the lesser wing of the sphenoid is not
,developed, so that the optic foramen and superior orbital fissure are confluent.
But the anterior boundary of the common opening is formed as usual, below by the
presphenoid, which is broad from side to side but shallow vertically, and anteriorly
.and above by its lesser wing. As the optic nerve emerges (the course is marked in
fig. 32 by an arrow) it is separated from the posterior recess of the nasal c a ~ t y ,
which contains the last ethmo-turbinal, by the orbital surface of the frontal bone,
.and from the capacious naso-pharyngeal duct by the orbital plate of the palatbe.
The skull base (fig. 33) shows the wide presphenoid articulating behind with the
basi-sphenoid and joined anteriorly to the ethmoid. The lesser wings form between
them a prominent shelf which overhangs the deep chiasmatic sulcus (between the
markers ‘ P.S.’ and ‘ O.S.’).
41*
696
R. WHEELER HAINES
:
INTERORBITAL
SEPTUM IN THE CHONDROCRANIUM.
I n contrast to the septum of adult animals, that of embryos has been discussed
exhaustively by a number of authors, particularly Matthes (1922), de Beer and
Woodger (1930) and de Beer (1937). In the present survey there wiII be no need to
repeat this work, and precise references will only be given where there is some specid
reaeon for doing so.
I n all mammals the anterior end of the central stem of the chondrocranium is
laterally compressed so as to form the nasal septum. I n some embryos the compression stops at the level of the posterior cupdm of the nasal capsules, in which case
there is no interorbital septum (figs. 34 A and C, and 36 A), but in other embryos
the compression is continued posteriorly into the orbital region, and in this case a n
interorbital septum is said to be present (figs. 34 B and F, and 36 B). This definition
FIG. 34.-Chondrocrania of mammals, transverse sections through orbital region.
A. Dasyurwr (Broom, 1909).
E. Megaptem (Honigman, 1917).
F. Oryctolagus (Voit, 1909).
B. Didelphys (Toeplitz, 1920).
G. Equua (Limberger, 1926).
C. Sorez (de Beer, 1937).
H. Szcs (Mead, 1909).
D. Halicore (Matthes, 1921).
agrees with that of Matthes (1922, p. 159), who understood by a typical septum
' eine senkrecht gestellte, mehr hohe als breite, unpaare Knorpelplatte, die als freies
Schaltstuck zwischen hinterer Nasenkapsel und Pars chiasmatica der Orbitotemporal
region (characterisiert durch Ansatz des Orbitalfliigels) steht '. The emphasis on
lateral compression is important, for writers on the region have not always followed
the same procedure in their descriptions, and this has led to difficulties in understanding their work.
Septa are more frequent in embryos than in adults. In monotremes they are
found in Omithorhynchus, but not in Echidna, and in marsupials in Didelphye (fig.
34 B) and Perameles (Cords, 1914, p. 49), but not in Caluromys, Dasyurwr (fig. 34 A)
or Trichosurus. They are not found in any typical insectivores or chrysochlorida.
THE INTERORBITAL SEPTUM IN MAM,VALS
597
(Roux, 1947), of which Erinaceus, T a l p , Sorm (fig. 34C), Sumus, Ekphantulus,
Setifer, Eremitulp and Chrysochloris have been described, in the bat Minioptmu
or in Galeopithecus.
In edentates Fawcett’s (1921) 12mm. Tatusia has a distinct keel on the under
aurface of the central stem opposite the preoptic roots, though the formation has
disappeared in his 17 mm. embryo, and Weber’s (1927) figure of Bradypm has the
septum labelled, but in neither case are sections given, so that the presence of a
septum is uncertain. The sirenian Halicore (Matthes, 1921) has a wide, low keel
(Sg. 34 D), but no definite septum, and it is absent, too, in Manatua (Hirschfelder,
1936). I n the whales, Lagenorhynchus, Balaenoptera, P h e w and Megaptma
(fig. 3 4 E ) , the central stem may be keeled to support the rostrum, but there is
no true septum, though de Beer and Woodger (1930) give it as ‘ feebly represented ’.
FIO.35.-Chondrocrania of lemurs and primates, transverse sections through orbital mgion.
E. Ateles (Frets, 1913).
A. Lemur (Henckel, 1928).
F. Alouattu (Frets, 1913).
B. Propitheem (Frets, 1914).
C. TUrEiU8 (Frets, 1914).
G . Semnopithecw (Frets, 1914).
D. Cullithriz (Henckel, 1928).
H. Hom.0 (Frets, 1914).
I n carnivores de Beer and Woodger give the septum as absent in Fdia, but
Terry (1917, p. 318) describes the stem as becoming ‘ thickened opposite the optio
foramina and finally triangular in its most anterior part. Here the two extracranial surfaces incline medially and ventrally to meet in a keel, forming a short
interorbital septum which passes forward into the septum nasi ’.
I n Canis de Beer and Woodger give the septum as ‘ feebly represented ’, but it is
as wide as it is high (Olmstead, 1911). Presumably the septum would be found in
the embryos of Herpepestes and other forms in which it is present in the adult, for
though the embryonic septum may be lost in the adult, the reverse is unknown.
598
R. WHEELER HAINES
:
I n the seal Poikilophoca de Beer cites the septum as ‘ absent ’, but Fawcett (1918,
p. 417) describes an ‘ interorbital septum ’ which ‘ somewhat triangular in coronal
section behind, changes anteriorly to a pear-shaped outline with the large end
downwards’. Indeed, such septa might be expected in the embryos of typical
seals, since they are found in the adults of the eared types.
I n rodents septa have been found in all three genera examined, Oryctohgus
(fig. 34 F), Xerus and Arvicola (Microtus),though in the last two they are lost in the
adult. I n the ungulates the septum is found in Bos, also quite definitely in Equus
(fig. 34 G ) , though de Beer and Woodger give it as ‘ absent ’. The chondrocranium
of Ovis has not been sectioned, but Decker (1883) figures a definite septum. Mead’s
paper on #us has often been overlooked, but his description (p. 193) is delinite. ‘ The
basal portion of the regio orbitotemporalis, which lies in front of the sella turcica,
is higher than wide and so forms a vertical plate, a true interorbital septum. Near
its anterior end, where it joins the nasal septum, it is 29 times as high as broad ’.
His figure, too (fig. 34 H in this paper), shows the septum better developed than in
any embryo known outside the primates. The chondrocrania of the smaller deer
and antelopes have not been examined, but there can be no doubt that such a form a5
Neotragus would show a good septum, for the presphenoid is ossified in cartilage,
and indeed some of the cartilage still persists in the adult a t the lower margin of the
septum (fig. 9). In the hyracoid Procavia, also, a short interorbital septum is present
in the embryo (Lindahl, 1948). Taking the embryonic and adult structure together,
the ungulates must be admitted to a place among the groups in which the septum
may be well developed.
Among tupaiids the septum is shown distinctly in the embryo Tupaia by Henckel
(1928), though de Beer and Woodger (1930) give it as ‘ absent ’ ; it has also been
found in all the embryos of lemurs examined, Lemur (fig. 35A), Nycticebus and
Propithecus (fig. 35 B). The septum is large in Tarsius (fig. 35 C ) and in the New
World monkeys Callithrix (fig. 35 D), Chrysothrix and Ateles (fig. 35 E). It is absent
in Alouatta (fig. 35 F), for the central stem is wide in preparation for pneumatization.
It has also been recorded as absent in Cebus (Henckel, 1928))but here, as it is variable
in the adult, it is probably so in the embryo, and Frets’s (1913) most posterior section
shows a deep septum persisting in the region of transition between the nasal and
interorbital regions. I n Old World monkeys the septum is found in Macucus,
8emmpithmus (fig. 35 G ) and Nasalis embryos. I n Homo it appears in the embryo
as a thin sheet in the early stages (fig. 35 H), but later it becomes thickened (Jacobson,
1928) and fhally pneumatized.
FACTORS
INFLUENCINU
THE PRESENCE
OF THE SEPTUM.
Taking the mammals as a whole the interorbital septum is more likely to be
developed in animals with large eyes rather than small eyes (ungulates and primates
versus insectivores and bats), in young animals rather than old (Omithorhynchus
and Didelphis embryos versus adults), in small animals rather than large (Cephulophorus coeruleus and Herpestes griseus versus C . huweyi and H . brachyuw), in
animals with long narrow skulls rather than short broad skulls (Rhinosciuw and
Poianu versus Sciurus and Crypbpocta),in animals with the nasal apparatus poorly
developed rather than well developed (eared seals and primates versus typical
carnivores and lemurs), and in animals with simple nasal cavities rather than those
provided with accessory air sinuses (Madopwc, Saimiri and M w c u s versus Owis,
Alouatta and Homo).
All these factors influence the width of the orbit relatively to that of the skull
in the orbital region, and make it more likely that the orbits will become contiguous
so that they are separated by a thin septum only. The eyes and their accessory
apparatus do not necessarily fill the orbit, for much of the space may be occupied
by fat, but large eyes necessarily imply large orbits to accommodate them. The eyes
show pronounced disharmony in growth, for they are relatively much larger in the
embryo and young than in the adult, and even though they usually bulge from the
THE INTERORBITAL SEPTTJM IN MAMMALS
599
head in the embryo the orbits are relatively larger. In any series of closely-related
animals, though the eyes will be found to be larger in the larger types, they do not
increase in size proportionately to the other parts (Ashley Montagu, 1944), so that
in the smaller members of the series they are disproportionately large. The orbits
must remain more or less spherical in shape even when the skull as a whole elongates,
so that when this happens the orbits become relatively larger as the skull narrows
down. Withdrawal of the nasal apparatus in animals which have become microsmatic, whether through marine or through arboreal life, is likely to bring the orbits
into contiguity. Development of air sinuses in the presphenoid may, on the other
hand, be secondary to loss of the septum, for they are found only in large animals
whose orbits have become separated.
MORPHOLOGY.
The condition of the interorbital region of the anterior brain-case in nonmammalian tetrapods has recently been reviewed by Save-Sodebergh (1947) and
Bellairs (1949). In most modern reptiles a well developed cartilaginous interorbital
septum is present in both the embryonic and adult conditions; in snakes and
amphisbaenids, however, the septum appears to have been secondarily lost. There
is some evidence that a cartilaginous or partly ossified septum was present in
cotylosaurs, and that this structure became more extensively ossified in the pelycosaurs and therapsids (Romer and Price, 1940; Olson, 1944). These findings
relate to the adult anatomy, but it may be assumed that, as in L?pAenodun, lizards
and crocodiles, an unossified septum was also present in the embryos of these extinct
forms.
Conditions in the earliest mammals are not known but the osteology of many
Paleocene forms has been worked out in some detail (Matthew, 1937), and no
occurrence of a septum has been reported. This negative evidence, together with the
fact that the interorbital septum is absent in the adults of the most primitive living
mammals (monotremes, marsupials, insectivores) strongly suggests that its presence
in more advanced forms is secondary.
The septum of the typical reptilian chondrocranium is, like that of mammals,
continuous anteriorly with the nasal septum, and extends back to form the common
anterior boundary of the optic foramina. Its upper border is related on either side
to the planum supraseptale of the orbital cartilage. This cartilage, which is chondrified from an independent centre in the mesenchymatous brain capsule, is a t first
separated from the septum by an unchondrified cleft, but later in development fuses
with it, and, a t least in some forms, contributes tissue to it. Anteriorly the septum
is separated from the spheno-ethmoidal commissure, by the orbito-nasal fissure.
Below, since there is no naso-pharyngeal duct, the septum abuts freely against the
mucous membrane of the mouth. I n many forms one or more fenestrae are developed
in the septum as development proceeds, but these are secondary deficiencius filled
in by membrane, which takes the place of the resorbed cartilage (Bellairs, 1949).
I n mammals with septate chondrocrania (fig. 36 B, 38 D) the general relationships
are maintained, but there are great changes in proportions. The planum supraseptale forms the ala orbitalis, and is still attached to the nasal capsule by a sphenoethmoidal commissure. But the capsule itself is greatly expanded to include the
new ethmo-turbinal apparatus of the nasal cavities. The orbital region of the
chondrocranium, with its septum, is shortened, so that the optic foramina and orbitonasal fissure come near together, narrowing down the originally extensive attachment
of the supra-septa1 cartilage to the septum in reptiles to give the relatively -mow
' pre-optic root ' of mammals.
Fischer (1901) and others have suggested that the lateral walls of the nasal
capsules migrated backwards in mammals so as to include a part of the original
interorbital septum within the nasal cavity, leaving less for the interorbital part,
and there is some evidence for this view, as the interorbital region of the central
stem is reduced in the older, as compared to the younger, chondrocranium of Talpa.
600
R. WHEELER m E S
:
But since the lateral wall is attached to the septum behind the cranio-nasal passage
for the olfactory nerve, so that the lateral and medial walls would naturally expand
together, and since the backward extension of the orbito-nasal fissure suggests
compression of the region behind it, it seems more reasonable to postuhte a simple
.expansion of the nose including its septum and a reduction of the orbital region
as major changes in mammalian evolution. Either view is compatible with the direct
derivation of the mammalian from the reptilian septum.
de Beer and Woodger (1930) found that in the rabbit the septum develops fiom
a n unpaired trabecular chondrification which appears at the 16 mm. stage (my fig. 37).
This cartilage expands to form a vertical sheet at 16.5 mm., and by 19 mm. has formed
a well developed septum continuous throughout the nasal and anterior orbital
regions. The anterior part of this sheet, that included between the nasal capsules,
forms the nasal septum, the more posterior part the interorbital septum. The
slae orbitales develop quite independently of the central stem, and are still, a t 19 mm.,
remote from the interorbital septum, so that the optic foramina and orbito-nasal
fissures are not yet separated off from each other. But at 19.5 mm. a bar of cartilage,
:h.
ala o r b i t aLis
~
_
-
MnmmaL
w i t h o u t Septumprcoptic rooc
c hiasm a
central stern
with
septum
FIG. BB.-Anterior parts of chondrocrenia of mmmale, without end with septum.
the pre-optic root, joins each ala to the postero-superior corner of the septum.
Since by this stage the ala has also become joined to.the nasal capsule by the ephenoethmoidal commissure and to the central stem behind the optic nerve by the postoptic root, the orbito-nasal fissure and the optic foramen are now complete.
I n the rabbit, then, the only animal in which the development of the septum
has been worked out, it is a direct derivative of the trabecular part of the stem.
In the typical reptiles the trabeculae are separate in the precartilaginous stage, a
primitive condition no longer found in the rabbit, but the main body of the septum
is developed from the fused trabeculae just as in the rabbit (Bellairs, 1949).
THE INTERORBITAL SEPTUM IN ,MAMMALS
601
Fischer (1903), on h d i n g septa in primate embryos, suggested that the primates
had left the primitive mammalian stock a t an early stage of their evolution, when
that stock still had septate chondrocrania, while the other mammalian groups had
progressed to complete obliteration of the septum. He did not, a t that early date,
realize how widespread septa are in mammalian embryos, and other authors have
not cared to commit themselves on the question of the primitive state. But taking
the evidence of distribution together with that of the close similarity in both the
:structure and development of the reptilian and mammalian septa, it seems likely
that the presence of some remains of the septum in the embryo is a primitive
mammalian feature rather than that it has been entirely lost and then regained in
a number of mammalian groups. I n animals such as Echidna, most marsupials,
"edentates, and insectivores where there is no septum, a series of features, more or
Jess pronounced in any particular instance, is usually found together (fig. 36A)
%IQ.
37.-Development of chondrocranium of rebbit (de Beer and Woodger, 1930 ; Voit, 1909).
'The nasal, capsules are large, the orbito-nasal fissure reduced to a narrow cleft or
partially obliterated, the central stem wide, the ala orbitalis narrow anteroposteriorly, and the pre- and post-optic roots thin, absent or joined together before
they are attached by a common root, so that the optic nerve pierces the a h instead
.of passing between it and the central stem. All these trends are away from reptilian
.conditions, so there is no dificulty in considering them as secondary, and the animals
in which they are developed as specialized in these respects.
Fischer's (1901) diagram (my fig. 38 A), comparing a lizard and a mole, shows
very clearly the expansion of the nasal capsule and brain and the reduction of the
orbital region of the central stem between them, with the displacement of the eye
on to the side of the nasal capsule, but the mole, with its small, laterally placed eyes,
is not a typical mammal. Muller's (1936) diagram (my fig. 38 B) shows the orbits
forced apart in the mammal, but suggests little of the anatomical structures involved.
'The attempt to summarize my conception of the region (fig. 38 C, D) includes the
dGasma and optic foramina as relevant details and shows the septum a~ disappearing
Tather than as being split open or included in the nasal cavity.
602
R. WHEELER HAINES :
As regards the persistence of the septum in the adult, Voit (1909) and others have
already suggested that its presence in the rabbit is correlated with the large size
of the eyes, and the sporadic occurrence of the septum in other rodents and carnivores
can usually be associatedwith the delicate visual control of leaping, h u n t i i and other
activities. The smaller living ungulates live in the thick undergrowth of forests,
and are crepuscular in habit. Their eyes are very large and provided with tapeta,
which increase sensitivity a t low illuminations a t the expense of acuity, and such
eyes can only be accommodated in small skulls by bringing the orbits together and
SO forming an interorbital septum. Since it is reasonable to believe that the ancestral
ungulates were small, forest-living animals, it seems probable, that the septa are-
brain
Mole
I
qas
an tor b i t aLe
oramen
mammal
FIG.38.-Diagrams of chondrocrania. A : Fischer (1901),reptile and mole. B : Muller (1935),
reptile and mammal. C, D : Present author, mammal with (D) and without (C) septum.
primitive features of the group. In the larger types the growth of the skull as a
whole has separated the orbits, so that though the septa may be well developed in
the embryo they are lost in the adult. Further, in late embryonic life, extra ethmoturbinals beyond the ordinary mammalian quota and a complicated system of air.
sinuses are added to the nasal apparatus (cp. Vogler, 1926, 28 om. pig embryo, and
Zuckerkandl, 1887, adult pig), and these come to occupy the space between the
orbits.
I n primates all authors are agreed that the septum is a primitive feature. The.
separation of the orbits in some of the larger forms probably depends partly on the,
growth of the skull as a whole, and partly on the expansion of the brain.
THE IXTERORBJTAL SEPTUM I?X X A M U L S
603
FLOOR OF THE SKULL.
The ethmoid, presphenoid and frontal bones all help to build up the floor of the
brain case in the orbital region, but their exact arrangement is variable. The
ethmoid may articulate with the presphenoid on the surface of the floor, e.g. in
Hemiechinus (fig. 39) or the frontals may pass behind the ethmoid so as to articulate
with each other, cutting off the ethmoid from the presphenoid, e.g. in Oryctolugus
(fig. 5 ) .
Wood Jones (1929) suggested that the ethmoid-presphenoid contact was:typical
of mammals in general (e.g. marsupials), whereas the frontal extensions were
characteristic of monkeys and anthropoid apes (‘ with the single variable exception of
the orang ’), as opposed to man, who had retained the primitive contact. So, he
argued, man was free from this simian specialization,and could not have been derived
from a simian type of ancestor. Ashley Montagu (1930) has criticized the argument,
and has recently (1943) published a careful summary of the facts based on an
FIG. 39.-Hemiechinus
auritus (Kamal Wassif), skull base.
adequate series of skulls. It now appears that the anthropoid apes are much more
variable than had been supposed, for he found the ethmoid-presphenoid contact
in all orangs, most chimpanzees and some gorillas. But neither Ashley Montagu
nor any other author has attempted any morphological explanation of these
variations.
Now, in marsupials (figs. 32, 33) and insectivores the body of the presphenoid is
stout, for it encloses the nasal cavities as they extend back to the region of the optic
foramina (cp. Wood Jones’s Dasycercus and my fig. 6) and in this case it may be
widely exposed on the floor of the skull, and the frontals thereby be excluded from
contact with each other in this region (fig. 40A). But where the body of the
presphenoid forms a narrow interorbital septum the frontal bones may grow in to
articulate with its upper margin, and a t the same time with each other, SO that
the presphenoid is excluded from this part of the floor (fig.40 B). When the frontal
bones themselves help to form the septum then they must necessarily meet in the
cranial floor assthey turn ventrally into the septum (fig.40 C).
604
R. WHEELER HAINES :
It has been shown that in most New World and Old World monkeys there is a
well developed septum, so that, as would be expected, the frontals separate the ethmoid
and presphenoid. Exceptions may be anticipated only in the relatively few forms
where the septum has been lost owing to the inflation of the presphenoid by air
sinuses, and indeed the only records of ethmoid-presphenoidcontacts are in Alouatta
(Ashley Montagu, 1943), a peculiar form which no morphologist would consider
as illustrating primitive conditions. Similarly the presence of the contact in
anthropoid apes and man is correlated with the inflation of the presphenoid by
highly developed air sinuses, and the condition of the skull base, which is but the
surface effect of the underlying structure, must be considered a secondary reversion
and not truly primitive. I n fact, the evidence of the orbital region can be taken as
supporting the close relationship of the anthropoid apes and man rather than the
xeverse.
cranial
cavitv
.
FIG.40.-Structure
of orbital region and skull base.
IRREVERSIBILITY
OF EVOLUTION.
The development of the conception of irreversibility of evolution has been
.carefully reviewed by Petronievics (1919). Dollo’s f i s t enunciation, in 1893, ‘ Un
.organisme ne peut retourner, m6me partiellement, A un &at anterieur, d6jA r6alis6
dam la s6rie de anc6tres ’, referred to the organism as a whole, and his later statement
of the law merely elaborates his conception. ‘ Un organisme ne reprend jarnab
.exactement un &at anthrieur, m6me s’il se trouve place dans des Conditions de 1’Exiatence identiques A celles qu’il a traverse. Mais, en vertu de l’Indestructibilit6 du
Passe, il garde toujours quelque trace des &apes intermediaires qu’il a parcouru
But Dollo’s examples, the pseudo-dentition of an Eocene bird, the pelvis of dinosaurs,
the tentacular arms of octopods, etc., refer, not to organisms as a whole but to parts
,of organisms. The modern conception of reversibility is represented more closely
by Arber’s (1919 a) ‘ general rule that a structure or organ once lost in the course of
phylogeny can never be regained: if the organism subsequently has occasion to
replace it, it cannot be reproduced, but must be constructed afresh in some different
mode though this rule was formulated independently on botanical evidence.
Needham (1938) has extended the formulation to include physiological as well m
morphological traits, ‘ Evolution is reversible in that structures or functions once
gained may be lost, but irreversible in that structures or functions once lost can
never be regained ’, quoting as examples the enzymes of bacteria, the physiological
uraemia of elasmobranchs, the nitrogen metabolism of molluscs, and so forth.
’.
),
THE MTERORBITAL SEPTUM IN MAMMALS
605
Now, Arber (1919 b) collected together a number of examples of reappearances
of lost organs, some of which have been put forward as exceptions to the ‘ law ’,
including the reappearance of stamens and ovary chambers in plants and of toes in
mammals, and Cave and Haines (1944) have added ethmoturbinals in man. But,
as Arber pointed out, these organs are all members of series, and their reappearance
can be comidered as examples of meristic variation.
Reversds of evolutionary trends are generally accepted by palaeontologists, for
example the reduction of the lower jaw in the later elephants and the shortening of
the legs in forest horses, and Petronievics (1919) suggested that ‘ if a part degenerates,
but if the disposition of the germen is not too enfeebled, it can be followed by a
progressive evolution ’, a view elaborated by Muller (1939). Gregory (1935) quoted
several instances from primate evolution where the ‘ law ’ of irreversibility had been
applied too strictly in ruling out possible phylogenies involving minor reversala.
of trend. Matthew (1937, p. 321) went further, for he suggested that as regards
individual parts complete reversibility wa8 possible, given that they were ‘ of such
simple form that the end results of a re-adaption could not be practically distinguished
from the primary adaption and quoted as examples the tritubercular external
form of the teeth in certain seals and whales (cp. my fig. 14).
The interorbital septum is a clear example of Matthew’s type of reversibility
of simple adaptions. It was developed in the ancestral reptiles, lost in early mammals,
redeveloped in typical primates and small ungulates, and lost again in some monkeys,
anthropoid apes and man, and in the larger ungulates. It is a simple structure
dependent for its existence on the arrangement of other structures in its neighbourhood, its ‘ germen ’ is always found in the embryo as a part of the central stem of the
chondrocranium, and there is no reason why it should not disappear or reappear in
the course of evolution. The ethmoid-presphenoid contact, present in typical
mammals and presumably in early types, lost in typical primates, and regained in
some monkeys and anthropoids and in man, is another example of the same kind.
’?
SUMMARY.
1. The distribution, relationships and structure of the adult interorbital septum
are described and figured in all groups of mammals known to possess it.
2. These include several carnivores and ungulates, besides the eared seals, rodents
and primates, where septa were known previously.
3. I n primitive mammals septa were not found in the adult skull, but may have
occurred in the chondrocranium of the embryo.
4. Possession of a septum in the adult is a primitive feature of the ungulates and
primates. I n both groups the septum may be lost in later evolution.
5. The structure of the base of the skull is correlated with that of the septum,
for when the frontal bones take part in the septum they must meet each other behind
the cribriform plate of the ethmoid.
6. Evolution is reversible in the case of such simple adaptions as the interorbital
septum or the ethmoid-presphenoid bony contact.
My thanks are due for the loan of material in the British Museum (B.M.), the
Museum of Vertebrate Anatomy, Cambridge (Camb.), St. Thomas’s Hospital Medical
School (Thos.), the London Zoological Society (Zool. SOC.),the London Hospital
and Dr. Kamal Wassif and Dr. Bulgakow of Fouad I University, Abbassia, Cairo,
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EXPLANATION OF LETTERING.
- - - - -- --- - - --
A line of dots and dashes ( - several of the fi-s.
and a line of dashes I.duct.-- The in6rorbital septum is stippied.
a h hypoch.
ale hypochiasmetica.
A.S.
alisphenoid.
B.S.
basisphenoid.
comm. orb. p w . c o d s s u r a orbito-parietalis.
comm. aphen. eth. commissura spheno-ethmoidalis.
Cr.Cv.
cranial cavity.
crista trans.
crista transversalis.
cupola post.
posterior cupola.
E.
ethmoid bone.
P.
frontal bone.
fis. orb. ma.,
Jias. orbito-nas. orbito-nasal fissure.
optic foramen.
F.O., for. opt.
frontal sinus.
P.S.
i.o.s., interorb. interorbital septum.
sept.
'internal pterygoid plate.
I.P.P.
inferior turbinal.
I.T.
lacrymal bone.
L.
lacrymal sac.
Lac. sac.
masseter muscle.
Mas.
~ i i i a .
M.
nasal bone.
N.
nasal capsule.
nasal cap.
nasal cavity.
Naa. caw.
}
) indicates the extent of the cranial cavity in
-)
that of the nasal cavity and nasopheryngeal
N.P.D.
nasal sept.
O.M.
O.S.
P.
parach.
preopt. rt.
postopt. rt.
perf.
P.M.
P.S.
Pl.
Pt
.
R.L.
R.M.
Sin. max.
8.0.
sq.
T.
Th.
T r . C.
trabr rudt
V.
2.
I, II,III, IV.
.
.
nasopharynged duct.
nasal septum.
orbital muscle.
orbitosphenoid.
parietal bone.
perachordal.
preoptic root.
postoptic root.
perforation.
premaxilla.
presphenoid.
palatine
pterygoid bone or mwcle.
rectus leteralis muscle.
rectus medialis muscle.
maxillary sinus.
superior oblique muscle.
squamosal bone.
temporal bone or muscle.
tooth.
trabecule cranii.
trabecular rudiment.
vomer.
zygomatic bone.
ethmoturbinah.
.