124: RESPIRATION. 



thing like the club on a playing-card, with middle, right, and left leaflets. The remain- 

 der of the organ is composed of radiating fibres of 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 par- 

 tially closed when the diaphragm contracts in inspiration, as the fibres simply surround 

 the tube, and none are attached to it. 



FIG. 42. Diaphragm. (Sappey.) 



1, 2, 3, central ten don ; 4, right pillar ; 5, left pillar ; 6, 7, processes "between the pillars ; 8, 8, openings for the splanch- 

 nic nerves ; 9, fibrous arch passing over the psoas magnus ; 10, fibrous arch passing over the quadratus lumbo- 

 rum ; 11, muscular fibres arising from these two arches ; 12, 12, muscular fibres arising from the lower six ribs ; 

 13, fibres from the ensiform cartilage; 14, opening for the vena cava ; 15, opening for the oasophagus ; 16, open- 

 ing for the aorta; 17, 17, part of the transversalis muscle; 18, 18, aponeurosis; 19, 19, quadratus lumborum; 

 20, 20, psoas magnus; 21, fourth lumbar vertebra. 



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 contrac- 

 tion has a tendency rather to increase than to diminish the caliber of the vessel. 



The orifice for the vena cava is surrounded entirely by tendinous structure, and con- 

 traction of the diaphragm, although 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 vivisec- 

 tions. 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 por- 

 tion and the contraction of the muscular fibres. The action of this muscle may be ren- 

 dered more apparent by compressing the walls of the chest with the hands, so as to 

 interfere somewhat with the movements of the ribs. By putting a strong ligature around 

 the spinal column and soft parts just below the diaphragm and cutting off the lower 

 half of the body, as was done by the assistant to the chair of physiology in the Bellevue 

 Hospital Medical College, Dr. 0. F. Roberts, the movements of the diaphragm may be 

 very beautifully exhibited in class-demonstrations. 



In ordinary respiration, the descent of the diaphragm and its approximation to a 

 plane are the chief phenomena observed ; but, as there is a slight resistance to the depres- 

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



