160 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



of the influence which the secretion of these glands 

 has on starchy foods in ourselves, it is often thought 

 that therein lies their prime function. Certain con- 

 siderations seem to show that this is not a correct 

 view ; in the Cetacea and other aquatic forms, and in 

 the blood-sucking Desmodus, the glands are con- 

 siderably reduced in size ; the dog " bolts " his food, 

 or, in other words, does not subject it to the influence, 

 which is not exerted in a moment, of the salivary 

 secretions ; while the kangaroo, which dwells in the 

 arid plains of Australia, has some of the glands of 

 great size. It would seem, therefore, that the prime 

 function of these structures is to afford a supply of 

 water for the solid food, and a further physiological 

 advantage is gained by the fact of this water being 

 supplied at or near to the temperature of the mam- 

 malian body. 



The well-known fact in human physiology that 

 the secretion of the parotids is the most watery, and 

 that of the submaxillary and sublinguals more viscid, 

 is paralleled in comparative anatomy by the large size 

 of the parotids in animals, such as ruminants, and 

 other herbivorous forms, which masticate dry foods ; 

 and the great size of the submaxillary in such 

 animals as the Echidna, or the ant-eater, which 

 require a viscid fluid for the purpose of catching their 

 insect prey. In the last-mentioned mammal some of 

 the fibres of the stylo-hyoid muscle encircle the sub- 

 maxillary ducts, which are thereby constricted when 

 the muscle contracts, and in this way the ejection of 

 the fluid is assisted (W. A. Forbes). 



With the exception of the buccal cavity, the greater 

 part of the enteric tract of a Vertebrate is developed 

 from the archeiiteron, the proctodeal portion being, as 

 a rule, exceedingly short. The general tract may be 

 conveniently divided into the oesophagus, stomach, 

 intestine, and rectal portion ; this last, in all except 



