THE DIAPHRAGM. ^71 



sist of a thin circular expansion, occupying the middle of 

 the larger muscle, and uniting that with the lesser. Through 

 the muscle are seen three remarkable openings, an upper 

 one in the interspace between the crura for the passage of 

 the aorta ; one a little lower, formed by the decussation of 

 the crura for the oesophagus ; and the third, or lower one, 

 perforating the cordiform, or heart-shaped tendon, for the 

 reception of the posterior vena-cava. 



The diaphragm is the chief, if not the sole agent, in 

 respiration ; it acts in opposition to the abdominal muscles, 

 which are the chief expiratory powers. By the contraction 

 of its radiated fibres, with the assistance of that of the 

 crura, the cordiform tendon is transformed to a plane sur- 

 face, and the dimensions of the chest from front to back 

 thereby considerably augmented. When this muscle acts, 

 in consequence of the shortening of its fibres, it loses its 

 convexity, as above stated, and the chest being thereby 

 enlarged as well as the lungs, the air rushes in, and inspira- 

 tion is performed. This muscle also assists in the natural 

 constant motion of the bowels, and lends its powerful aid 

 in expelling the faeces and urine ; and in females, facilitates 

 the birth of the young animal. 



The membrane by which the diaphragm is covered is very 

 liable to inflammatory attacks. In all cases of disease of 

 the lungs and bowels, the diaphragm is almost certain to 

 become inflamed and attended with considerable irrita- 

 bility ; and this is the cause of the breathing of the horse 

 being so much affected during inflammation of the chest and 

 abdomen. It is likewise concerned in coughing, yawning, 

 and sighing. Sometimes it is ruptured, occasioned by any 

 violent exertion. We are, however, unable to give distinct 

 indications of this condition. But no instance is known of 

 the animal surviving this malady. In cases of small rup- 



