178 CONTENTS OF THE CHEST. 



Since that period, from the increasing 1 and very proper habit of examining everj 

 dead horse, cases of this accident have rapidly multiplied. It seems that it .nay 

 follow any act of extraordinary exertion, and efforts of every kind, particularly on a full 

 stomach, or when the bowels are distended with green <& other food likely to generate 

 gas.* Considerable caution, however, should be exercised when much gaseous fluid 

 is present; for the bowels may be distended, and forced against the diaphragm to 

 such a degree, as to threaten to burst. 



An interesting case of rupture of the diaphragm was related by Professor Spooner, 

 at one of the meetings of the Veterinary Medical Association. A horse having been 

 saddled and bridled for riding, was turned in his stall and fastened by the bit-straps. 

 Something frightened him he reared, broke the bit-s-trap, and fell backward. On the 

 following morning, he was evidently in great pain, kicking, heaving, and occasionally 

 lying down. Mr. S. was sent for to examine him, but was not told of the event of 

 the preceding day. He considered it to be a case of enteritis, and treated it accord- 

 ingly. He bled him largely, and, in the course of the day, the horse appeared to be 

 decidedly better, every symptom of pain having vanished. The horse was more 

 lively he ate with appetite, but his bowels remained constipated. 



On the following day there was a fearful change. The animal was suffering sadly 

 the breathing was laborious, and the membrane of the nose intensely red, as if it 

 were more a case of inflammation of the lungs than of the bowels. The bowels 

 were still constipated. The patient was bled and physicked again, but without 

 avail. He died ; and there was found rupture of the diaphragm, protrusion of intes- 

 tine into the thoracic cavity, and extensive pleural and peritoneal inflammation. 



In rupture of the diaphragm, the horse usually sits on his haunches, like a dog; 

 but this is far from being an infallible symptom of the disease. It accompanies 

 introsusception, as well as rupture of the diaphragm. The weight of the intestines 

 may poasibly cause any protruded part of them to descend again into the abdomen. 



This muscle, so important in its office, is plentifully supplied with blood-vessels. 

 As the posterior aorta passes beneath the crura of the diaphragm, it gives out some- 

 times a single vessel which soon bifurcates; sometimes two branches, which speedily 

 plunge into the appendices or crura, while numerous small vessels, escaping from 

 them, spread over -the central tendinous expansion. As the larger muscle of tha 

 diaphragm springs from the sides and the base of the chest, it receives many ramifica- 

 tions from the internal pectoral, derived from the anterior aorta ; but more from the 

 posterior intercostal s which spring from the posterior aorta. 



The veins of the diaphragm belong exclusively to the posterior vena cava. There 

 are usually three on either side ; but they may be best referred to two chief trunks 

 which come from the circumference of the diaphragm, converge towards the centre, 

 and run into the posterior cava as it passes through the tendinous expansion. 



The functional nerve of the diaphragm, or that from which it derives its principal 

 action, and which constitutes it a muscle of respiration, is the phrenic or diaphragmatic. 

 Although it does not proceed from that portion of the medulla oblongata which gives 

 rise to the glosso-pharyngeus and the par vagum, yet there is sufficient to induce us 

 to suspect that it arises from, and should be referred to, the lateral column between 

 the superior and inferior, the sensitive and motor nerves, and which may be evidently 

 traced from the pons varolii to the very termination of the spinal chord. 



The diaphragm is the main agent in the work of respiration. The other muscles 

 are mere auxiliaries, little needed in ordinary breathing, but affording the most 

 important assistance, when the breathing is more than usually hurried. The mecha- 

 nism of respiration may be thus explained : Let it be supposed that the lungs are in 

 a quiescent state. The act of expiration has been performed, and all is still. From 

 gome cause enveloped in mystery connected with the will, but independent of it- 

 some stimulus of an unexplained and unknown kind the phrenic <erve acts on the 

 diaphragm, and that muscle contracts ; and, by contracting, its convexity into the 

 chest is diminished, and the cavity of the chest is enlarged. At the same time, and 

 by some consentaneous influence, the intercostal muscles act with no great force, 

 indeed, in undisturbed breathing ; but, in proportion as they act, the ri'os rotate 01 

 their axes, their edges are thrown outward, and thus a twofold effect ensues : the 

 posterior margin of the chest is expanded, the cavity is plainly enlarged, and also, bj 

 the partial rctati >n of every rib, the cavity is still more increased. 



* Percivall's Hiopopathology, vol. ii., No. 1, p. 152. 



