; 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 membrane 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 convexity 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 diaphragm, 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 diaphragm on record.
The cavity of the thorax is lined by a membrane, termed
pleura
, which covers the surface of the lungs.