330 



COMPARATIVE PHYSIOLOGY. 



as the stomacli itself and the gullet, take part. Probably as a 

 result of the descent of the diaphragm and consequent diminu- 

 tion of the intrathoracic pressure, the ascent of the cud is as- 

 sisted by an aspiratory process. The returning food is pre- 

 vented from passing into the nasal chambers by co-ordinated 

 movements analogous to those of swallowing. The whole pro- 

 cess is reflex in the same sense as is deglutition. 



Normally the rumen always contains considerable liquid, a 

 portion of which passes up with the cud, but is in great part 

 returned at once. A ruminant given dry food without water 

 can not return the cud. 



In the second mastication the process is in most ruminants 

 unilateral ; and as hundreds of cuds are to be chewed, a con- 

 siderable proportion of the whole day is occupied with rumina- 

 tion. When a single cud is sufficiently masticated it is swallowed, 



Fie. 373.— Stomach of dog (after Chaveau). A, oesophagus; B, pylorus. 



and. being finely comminuted passes at once through the small 

 opening between the reticulum and manyplies into the third 

 stomach, and thence into the abomasum, though possibly on 

 the way a little may pass into the first two stomachs. 



Pathological. — While moderate fullness of the paunch is 



