ANATOMY OF THE CHEST. 185 



CHAPTER XI. 



ANATOMY AND DISEASES OF THE CHEST : THE DIAPHRAGM J THE 

 PERICARDIUM ; THE HEART ; PLEURISY ; PNEUMONIA ; SPAS- 

 MODIC COUGH. 



THE chest is the superior, or in quadrupeds the anterior, cavity of the 

 trunk of the body : it is divided into two cavities by a membranous parti- 

 tion, termed mediastinum; and separated from the abdomen, or cavity 

 which contains the liver, spleen, pancreas, and other abdominal viscera, by 

 the diaphragm, which is of a musculo-membranous nature. This mem- 

 brane may be described, as it is divided, into the main circular muscle, with 

 its central tendinous expansion forming the lower part, and two appendices, 

 or crura, as they are termed from their peculiar shape, constituting its 

 superior portion. We trace the fleshy origin of the grand muscle, laterally 

 and inferiorly, commencing from the cartilage of the eighth rib anteriorly, 

 and following somewhat closely, as we proceed backward, the union of the 

 posterior ribs with their cartilages, excepting, however, the two last. The 

 attachment is peculiarly strong. It is denticulated : it encloses the whole 

 of the latter and inferior part of the chest as far as the sternum, where it is 

 connected with the ensiform cartilage. 



The diaphragm is the main agent, both in ordinary and extraordinary 

 respiration. In its quiescent state it presents its convex surface towards 

 the thorax, and its concave one towards the abdomen. The anterior con- 

 vexity abuts upon the lungs ; the posterior concavity is occupied by some 

 of the abdominal viscera. 



Thus far we have described the diaphragm as found in the horse, ox, and 

 sheep. There is some difference with regard to the dog. The muscular 

 part of the diaphragm is thick and strong in every species of dog, while the 

 aponeurotic expansion is comparatively smaller. From the smaller expanse 

 of the thorax of the dog, and the consequent little expansion of the dia- 

 phragm, the action, although occasionally rapid and violent for he is an 

 animal of speed is not so extensive, and more muscle and less tendon may 

 be given to him, not only without detriment, but with evident advantage. 

 Therefore, although we have occasional rupture of the heart of the dog, 

 oftener perhaps than in the horse, there is no case of rupture of the dia- 

 phragm on record. 



The cavity of the thorax is lined by a membrane, termed pleura, which 

 covers the surface of the lungs. 



The lungs on either side are enclosed in a separate and perfect bag, and 

 each lung has a distinct pleura. The heart lies under the left lung ; and, 

 more perfectly to cut off all injurious connexion or communication of 

 disease between the lungs and the heart, the heart is enclosed in a distinct 

 pleura or bag, termed the pericardium. This membrane closely invests the 

 heart, supports it in its situation, prevents too great dilatation when it is 

 gorged with blood, and too violent action when it is sometimes unduly sti- 

 mulated. Notwithstanding the confinement of the pericardium, the heart, 



