THE DIAPFPAOM. 
271 
gist of a thin circular expansion, occupying the middle ol 
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 
che 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- 
