ON THE RESPIRATORY FUNCTION OF THE INTERNAL INTERCOSTAL MUSCLES. By H. NEWETL
MARTIN, Professor of Biology in the Johns Hopkins University,
Baltimore, U. S. A., and EDWARD MUSSEY HARTWELL,
M.A. Plate I.
AN inspection of the ordinary text-books of Physiology is sufficient
to shew that the part played by the internal intercostal muscles, in
the production of the respiratory movements of the mamimal, is still
a subject upon which there is no agreement among Physiologists.
Thus in the text-books accessible to us we find these muscles, in some
cases with more or less reserve, described as inspiratory in function
(Dalton, Ludwig, Vierordt); or as both inspiratory and expiratory
in different portions (Carpenter, Flint, Hermann, Mc Kendrick);
or as expiratory only (Donders, Funke); while Foster says that
their action must at present be left an open question. This divergence of opinion induced us to attempt to solve the problem by a
method which, so far as we know, has not hitherto been employed.
To arrive at a decision as to the function of these muscles as rib
elevators or rib depressors, from a simple mechanical study of their
attachments seems impracticable; on account of the irregular shape
of the ribs and the doubt which must exist as to whether the upper
or the lower rib, to which one of these muscles is fixed, is to be
regarded as its origin or insertion. Moreover, if experiments be made
in which any or all of the other muscles be cut away, then direct
observation of the movement of the ribs which follows when the internal intercostals contract is useless so far as settling their function
goes; because we do not know that we have not removed some inuscles
which, in ordinary breathing, held fast either the upper or lower rib
and so determined either the inspiratory (rib elevating) or the expiratory (rib depressing) function of the muscle. The observation of
the result of direct electrical stimulation of these muscles is also not decisive: unless all the other muscles which work with them in breathing,
be also excited in proper order and degree; which is impossible. Unless this be done, however, we cannot by this means tell which rib
is normally the fixed one, when one of these muscles contracts. It
FUNCTION OF IVTERNAL INTERCOSTAL MUSCLES.
25
seemed to us, however, that, by isolating an internal intercostal and
then observing whether it contracted simultaneously or alternately with
the diaphragm, its function could be settled; since, from the general
co-ordination of muscular contractions in the respiratory movements,
there can be no doubt that muscles excited from the respiratory centre
and contracting during respiration simultaneously with the diaphragm,
are inspiratory muscles; and that those contracting when it relaxes,
are expiratory.
Dogs and cats were employed in our experiments. The animals
having been etherized, tracheotomy was performed and the apparatus
for artificial respiration connected with the wind-pipe. The abdomen
was opened by an incision along the linea alba and a transverse incision, so as to expose the diaphragm from below. The skin and the
serratus and pectoral and other muscles were then dissected away
from one side of the chest so as to lay bare the external intercostal
muscles from the fourth or fifth to the ninth or tenth ribs: except
where they were covered at their dorsal portions by the muscles
running alongside of the vertebral column. During this operation
several small vessels commonly required tying, especially in the dog.
One intercostal space, say that between the eighth and ninth ribs,
was then selected and the anterior part of the external intercostal
muscle divided, near its attachment to the lower of the two ribs, for
from an inch to an inch and a half at its sternal end. The internal
intercostal, which was carefully avoided during the operation, then
remained alone, with the pleura uniting the front parts of the two ribs.
The eighth and ninth costal cartilages and the tissues between them
were next divided, the chest opened and the artificial respiration
apparatus set at work. The tissues in the seventh and ninth intercostal spaces were then completely divided nearly all the way back
to the vertebral column.
Next, from the pleural side, a fine-bladed knife was inserted between the eighth intercostal nerve and the eighth rib near the vertebral column and an incision carried forwards, without cutting the
nerve, until it reached the outer end of the region where the external
intercostal muscle had been divided. In making this incision we have
found it impossible to avoid cutting the intercostal vein of the space
operated upon; the artery was sometimes divided also, but this did
not seriously impair the result. An incision of similar extent was then
made along the upper border of the ninth rib, and finally a bit of
both ribs corresponding in extent and position to these incisions w
26
H. NEWELL MARTIN AND E. 31. IHARTWELL.
completely cut away by bone forceps. By this means we obtained
from an inch to an inch and a half of the sternal ends of the eighth
an(I ninth ribs, united only by the internal intercostal muscle and the
pleura, and connected with the rest of the body by a band of tissues
consisting of the intercostal nerve (and artery) and some muscle
and pleura.
The remaining sternal piece of one of the ribs, sometimes the
upper, sometimes the lower, was then fixed in a clamp and placed
in such a position that the band of tissues above referred to hung quite
lax; so that movements of the ribs or even of the whole trunk, unless
unusually powerful, could not, through it, drag on the piece of muscle
to be experimented upon. A string was then attached to the other
rib and passed over a pulley to a lever which carried a weight and
extended the muscle. This lever carried a pen which wrote on the
paper of a Ludwig's Kymographion. A tambour was fixed beneath
the diaphragm and connected with another. This latter tambour was
provided with a lever which recorded the contractions of the diaphragm
immediately under the lever connected with the intercostal muscle.
The artificial respiration was then stopped, and the animal was
generally found apnceic. The further course of events differ in the dog,
and cat. In the former the diaphragm, when the apncea passed off,
made a few contractions without any activity of the intercostal muscle:
but this latter soon began to contract in regular alternation with the
diaphragm and before the occurrence of expiratory convulsions; in
fact with the commencement of dyspncea. Having made a few contractions, varying in number from fifteen to five or six, it again ceased
its activity, although the animal became more and more dyspnceic
as evinced by the contractions of the diaphragm. This cessation seems
due to the exhaustion of the muscle occurring rapidly from exposure
and the interference with its blood supply; since the contractions
become successively feebler before disappearing; and if artificial respiration be resumed and the animal kept alive until the muscle has had
a period of rest, then on again stopping the respiration the phenomena
are repeated, but the muscle makes a smaller number of contractions.
We have seen this restoration occur five times in the same dog. The
Figure, P1. I. to be read from right to left, shews the tracing of the fourth
set of contractions obtained in one experiment with the sixth internal
intercostal muscle of a dog; the ascents of the upper line indicate contractions of the intercostal muscle; and those of the lower line contractions of the diaphragm. It will be seen that they alternate perfectly.
FUNCTION OF INTERNAL INTERCOSTAL MUSCLES.
27
In order to get accurate tracings, we have found it necessary to divide
completely the straight and oblique muscles of the abdomen: otherwise
the action of these in dyspncea, as expiratory muscles, when pulling
down the ribs is apt to compress the abdomen and falsify the record of
the tambour beneath the diaphragm. In order to see the phenomenon,
however, it is not necessary to have any recording apparatus at all:
the alternating contractions of the diaphragm and the intercostal
muscle can be readily observed directly: and the latter occur in greater
vigour no doubt from the absence of the shock due to the extensive
abdominal incisions and the exposure of the viscera.
In the cat the internal intercostal muscle rarely contracts until the
occurrence of expiratory convulsions; but during these it contracts very
powerfully when the diaphragm is most completely relaxed. While the
convulsions occur, it is necessary to hold the vertebral column of the
animal very firmly in order to prevent any drag on the intercostal
nerve. Usually the muscle is exhausted during these contractions;
but on these convulsions passing off we have sometimes seen it
in action alternately with the first few of the slow final contractions
of the diaphragm; its contraction occurring immediately after the
diaphragmatic, and beino followed by a shorter or longer pause before
the next diaphragmatic contraction began.
In some experiments the strip of muscle used was the portion
between the, costal cartilages; which is not covered by the external
intercostal. The results thus obtained were the same as when a portion lyina between the bony ribs was employed. The results of our
experiments, we believe, shew decisively that the internal intercostal
muscles are expiratory in function throughout their whole extent; at
least in the dog and cat: and that in the former animal they are
almost " ordinary" muscles of respiration, coming into play very early
in dyspncea, while in the latter they are " extraordinary " respiratory
muscles, only active during extreme dyspncea; unless the later action
be ascribed to a diminution of the irritability of the muscle, brought
about by the unnatural conditions to which it was exposed, which may,
perhaps, influence more quickly the thinner muscles of the cat than the
thicker ones of the dog.
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