328 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



long as a quarter of an hour, and then the direction of mastication may- 

 be reversed, the change usually occurring as a new bolus enters the 

 mouth, and not during the mastication of one which has already entered 

 the mouth. In certain animals, as in the dromedary, the mastication of 

 rumination is alternate ; that is, the lower jaw moves first to the right and 

 then across the centre line to the left, and so on. In young animals the 

 rhythm of mastication is always more irregular than in adult animals ; 

 the number of strokes of the teeth to each bolus varies according to the 

 species of animals, the age, the character of the food, etc. Thus, dry 

 food requires more chewing than green food, perhaps thus explaining 

 the statement that animals ruminate more in winter than in summer. 

 Young animals have a smaller number of teeth than older animals, and 

 therefore require a longer time for mastication, as is also the case in old 

 animals, where the teeth have become imperfect. The rapidity of motion 

 of the second mastication closelj' corresponds in character with the other 

 motions of the same animal. In those animals which are habitually slow 

 and sluggish in their movements, as the ox, buffalo, etc., the movements 

 of mastication will be slower and more deliberate than in animals, such 

 as the antelope and gazelle, in which the muscular actions are rapidly 

 performed. Even in animals of the same species it will generally be 

 noticed that the movements are more rapid in youth than in old age. 

 Toward the end of the period of mastication of the cud, the movements 

 of the jaws become considerably accelerated. If rumination is interrupted 

 from any disturbance, the bolus is held in the mouth for a time and mas- 

 tication again completed before it is swallowed. Even when so inter- 

 rupted the average number of strokes of the teeth to each bolus is not 

 interfered with. If the disturbance is sufficiently severe to prevent the 

 resumption of rumination, the bolus is held in the mouth for some time 

 and then swallowed by several rapid movements of deglutition. During 

 the mastication of the cud the bolus does not pass between the incisor 

 teeth, but remains between the molars. As has been stated, the move- 

 ments of mastication constitute the principal stimulus to the secretion 

 of the parotid glands. It is these glands, therefore, which secrete most 

 actively during rumination, while the salivary glands of the anterior sys- 

 tem are almost inactive. In the ruminant animal the secretion of the 

 parotid is never entirely suppressed, for the fluid which it pours out is 

 essential to active rumination. Since, if a fistula be made of the parotid 

 ducts, and the parotid saliva conducted outside of the mouth, even al- 

 though the animal be supplied with water, rumination becomes more and 

 more difficult, and after three or four days becomes impossible. The 

 saliva is therefore essential to active rumination. When the food enters 

 the mouth the secretion of the parotid again becomes more active, and 

 these glands have been estimated to pour out nine hundred grammes of 



