DIGESTION 339 



Rumination is a reflex act. The centre has not been 

 located with certainty. It is probably situated in the 

 medulla. The two chief nerves involved are the phrenic 

 and the vagus. If the former be cut the diaphragm cannot 

 be contracted, but the food can still be regurgitated by a 

 more powerful contraction of the walls of the cavity and of the 

 abdomen. If the vagus be cut, the walls of the cavities are 

 paralysed and the process ceases. 



It is a remarkable fact that, though boluses can be 

 returned from the rumen to the mouth, the ox does not 

 vomit, even when distension of the rumen causes distress. 

 Why vomiting does not occur is unknown. It has been 

 suesrested that the vomiting centre in the medulla is un- 

 developed. 



(2) Digestive Changes in the Rumen. — The contents of the 

 rumen are subjected to a churning by the contraction of the 

 powerful muscular bands in the wall of the cavity. Newly 

 added food is therefore mixed with the previous contents. 

 No digestive juice is secreted, the only fluid added to the 

 food beinsr the alkaline saliva from the mouth. 



In this warm alkaline mass the fibrous substances become 

 softened and prepared for the further digestive processes. It 

 is probable that finely-divided material may pass direct from 

 the rumen to the omasum, though doubtless the greater bulk 

 is remasticated. 



The conditions in the rumen where the contents are 

 warm and alkaline favour the conversion of starch to maltose 

 by the enzyme ptyalin. Whether ptyalin is present in 

 ruminants, however, is doubtful, and the extent to which 

 the conversion takes place is unknown. 



Certain enzymes contained in the food may act. Cytase 

 has a feeble action on cellulose. Proteolytic enzymes may 

 act on protein, and amolytic enzymes on starches. These 

 changes due to enzymes contained in the food are, however, 

 of minor imj^ortance. 



Fats are freed by the disintegration of enclosing 

 substances, but in this compartment they undergo no 

 chemical changes. 



The rumen swarms with bacteria which attack the 



