268 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



They are, therefore, perfectly inert as regards any action to which they 

 may be subjected by the digestive juices. In the omnivora we find 

 mastication occupying a mean as regards importance between the her- 

 bivora and the carnivora. Where an omnivorous animal feeds on vegetable 

 diet the performance of mastication is as important as in the herbivora ; 

 while when on a meat or animal diet its importance becomes reduced to 

 the secondary degree in which it is seen in animals of a purely carnivo- 

 rous type. 



IV. DIGESTION IN THE MOUTH. 



The Salivary Secretion. — The salivary glands appear in most verte- 

 brates as tubular glands, as in insects, but in the mollusks they take on 

 the lobular form which characterizes them in the vertebrates. In birds 

 the salivary glands are small in the species which live on soft animal food 

 (waders and web-footed species), while they are larger in the graniverous 

 birds. In birds the saliva is mainly to assist in deglutition by lubricating 

 the food, as buccal mastication does not occur in these animals. In 

 certain birds, as the woodpecker, the saliva^' secretion assists in the 

 prehension of food. In the fishes, from the nature of their food, which 

 ■requiresno mastication, we find the salivary glands almost entirely absent, 

 even in such groups as the cetaceans which belong to the general di- 

 vision of mammals. Here, also, we find that their food requires no pre- 

 liminary subdivision before being swallowed. As has been already 

 mentioned, one of the uses of the saliva is to assist in mastication ; where, 

 therefore, mastication is not performed we have in such animals a corre- 

 spondingly rudimentary condition of the salivary glands. On the other 

 hand, in nearly all animals which possess buccal or pharyngeal teeth 

 there is usually a glandular apparatus whose secretion, by macerating the 

 food, is destined to facilitate mastication and deglutition. In a general 

 way it may be said that in most cases where there are permanent prehen- 

 sile organs salivary glands are also present (Letournean). Thus, the fly 

 emits on the particles it is about to draw in a brown liquid which dilutes 

 them. In reptiles the salivary glands become very highly developed , and 

 in certain of them their secretion acquires a poisonous character. In 

 chelonians and saurians the salivaiy apparatus consists principally of 

 lingual glands. In the chameleon they are located in the tongue and 

 secrete the sticky fluid which is of importance in their mode of prehen- 

 sion of food. Their maximum development is, however, reached in 

 mammals, with the exception of the cetaceans, as already alluded to, 

 where the lacrymal glands are also absent, and is especially marked in 

 the herbivora, whose food requires the finest comminution in the mouth. 

 In the ant-eater the salivary apparatus is enormously developed, the 

 glands covering the fore part of the neck and even extending to the chest, 



