Uscs.—The chief use of the diaphragm is to assist in the function of respiration, and it will be found to be the principal agent in the me chanical part of that process. By its action the thoracic cavity is enlarged from above downwards, whilst its circumference is in creased by the intercostal and other muscles.
\Then the diaphragm acts, the entire muscle descends, pushing the abdominal viscera down wards and forwards; but its different portions descend very unequally. The tendinous centre is nearly fixed, and the aura are incapable of much change of position ; it is only in the broad lateral expansions that the motion is very apparent. The muscular fibres of these when relaxed are pressed upwards, and present arches, convex to the thorax, and rising even above the tendon ; but when brought into ac tion, each fibre approaches to a right line, which runs obliquely down from the tendon to its point of insertion. Thus, instead of a great arch we have a number of inclined planes, very short in front, very long at the sides, and of intermediate length further back, all surmount ed by a tendinous platform. The base of the lung resting on the muscle descends with it; the liver, stomach, spleen, and all the moveable viscera of the abdomen are pressed downwards and forwards against the abdominal muscles. 1Vhen the diaphragm descends, therefore, in spiration takes place by the rush of air into the expanding thorax ; when it ascends expiration is the result, the air being forced out. In the former case the diaphragm is active ; in the latter it is completely passive, following the re siliency of the lungs, and pressed up by the action of the abdominal muscles on the viscera beneath.* The central tendon descends very little on account of its attachment to the peri cardium: descent here would be useless or worse; but the lateral portions on which the broad bases of the lungs rest, freely change their place, and allow of considerable expansion of the thorax where it is most required.
From viewing the insertion of the diaphragm into the lower ribs it might be thought that they would be drawn in by its action, and the capacity of the thorax thereby diminished more than increased ; but the intercostals prevent this occurrence by acting at the same moment to elevate and draw out the ribs.
The crura, besides acting in common with the broad muscle in enlarging the thorax, serve to fix the central tendon, and prevent it from being drawn to either side by the irregular ac tion of either half of the muscle, or forced too high up. They may also by their fibres conti nued on each side of the oesophageal orifice, and contracting in concert with the rest of the muscle, close that opening, and thus prevent regurgitation from the stomach at the time when this viscus is pressed upon by the descent of the diaphragm.
The extent to which the diaphragm descends is not great. The central tendon will not admit of much displacement in the normal state of the parts, and the shape and motions of the liver show that even the great alw do not un dergo much alteration. Bailer indeed says that he saw the diaphragm descend so much in violent inspiration as to present a convexity to wards the abdomcn.t But this is quite incre dible. The utmost muscular effort, if there were no fixed point in its centre, could only obliterate the arch ; but even this we think im possible on account of its attachments. We find on some occasions one side of the dia phragm act independently of the other.
The importance of the diaphragm in respira tion is shown by the difficulty with which that function is performed when the actions of the muscle are interfered with. Ascites and tu mours in the abdomen render the breathing shorter; even a full meal will have this effect, owing to the impediments to the descent of the diaphragm. If the phrenic nerves be divided in a living animal, grunt difficulty of breathing follows, the entire labour of respiration being thrown on the muscles which elevate the ribs. If the spinal marrow be divided above the giving off of the phrenic nerves, respiration ceases at once, hut not so if divided imme diately below that point ; and in a case of fatal dysplara Bedard could find no cause but a tumour on one of the phrenie nerves.