370 RESPIRATION. 



ence of the thorax, it mounts up into the cavity of the chest, 

 forming a vaulted arch or dome, with its concavity toward 

 the abdomen and its convexity toward the lungs. In the cen- 

 tral portion there is a tendon of considerable size, and shaped 

 something like the club on a playing card, with middle, 

 right, and left leaflets. The remainder of the organ is com- 

 posed of radiating fibres of the striped or voluntary muscular 

 tissue. The oesophagus, aorta, and inferior vena cava pass 

 through the diaphragm from the thoracic to the abdominal 

 cavity, by three openings. 



The opening for the oesophagus is surrounded by muscular 

 fibres, by which it is partially closed when the diaphragm 

 contracts in inspiration, as the fibres simply surround the 

 tube, and none are attached to it. 



The orifice for the aorta is bounded by the bone and 

 aponeurosis posteriorly, and in front by a fibrous band to 

 which the muscular fibres are attached ; so that their -con- 

 traction has rather a tendency to increase, than diminish, the 

 caliber of the vessel. 



The orifice for the vena cava is surrounded entirely by 

 tendinous structure, and contraction of the diaphragm, though 

 it might render the form of the orifice more nearly circular, 

 can have no effect upon its caliber. 



The action of the diaphragm can be easily studied in the 

 inferior animals by vivisections. If the abdomen of a cat, 

 which, from the conformation of the parts, is well adapted to 

 this experiment, be largely opened, we can observe the descent 

 of the tendinous portion, and the contraction of the muscular 

 fibres. The action of this muscle may be rendered more 

 apparent by compressing the walls of the chest with the hands, 

 so as to interfere somewhat with the movements of the ribs. 

 In ordinary respiration, the descent of the diaphragm and 

 its approximation to a plane is the chief phenomenon ob- 

 served ; but as there is a slight resistance to the depres- 

 sion of the central tendon, it is probable that there is also a 

 slight elevation of the inferior ribs, the diaphragm assisting, 



