SALIVAltY GLANDS OF MAMMALS. 409 



by the parotid of a horse is in direct ratio to the dryness of 

 the food and the difficulty experienced in its mechanical division* 

 The mastication of straw and hay causes a greater flow than does 

 that of oats and farinaceous matters ; the mastication of moist 

 forms of food hardly excites any. The saliva from the sublingual 

 and submaxillary ducts flows nearly in equal abundance whether 

 mastication be exerted on dry or moist forms of food ; and, owing 

 to its comparative tenacity, it is not easily imbibed into the centre 

 of the masticated material, but is gathered round the surface of 

 the mass, thus favouring its passage along the alimentary canal. 



The comparative anatomy of the salivary system supports the 

 conclusions of experimental physiology : thus, the parotids are re- 

 latively largest in mammals that masticate most ; the submaxil- 

 laries are largest in those that need the greatest amount of viscid 

 lubricating secretion. In the anteaters, hairy or spiny, the 

 parotid is so small as to have escaped the notice of Cuvier and 

 his continuators : l the submaxillary attains its maximum of size. 

 In many long-tongued Edentates (Myrmecophaga and Dasypus) 

 a bladder is superadded to the submaxillary gland both for storage 

 of a quantity of secretion needed in a sudden excess of outflow, 

 and also for adding to the tenacity of the secretion so poured out 

 to lubricate the tongue. In Echidna the end is gained by sub- 

 division with enlargement of Wharton's ducts. 



Most analyses of saliva have been made on that from the human 

 mouth which is the combination of the secretions of the various 

 glands above described. The peculiar animal principle called 

 6 ptyalin' is a nearly solid matter, adhesive, of a yellowish colour : 

 it is neither acid nor alkaline, is readily soluble in ether, alcohol, 

 and essential oils, but more sparingly soluble in water. It appears 

 to give the peculiar odour to saliva : when pure it may be kept long 

 at a moderate temperature without undergoing decomposition. 



Dr. Wright's analysis of human saliva 2 is as follows : — 



Water 988 



Ptyalin 1 



Fatty acid 



Chlorides of potassium and sodium 1 



Albumen combined with soda 



Phosphate of lime 



Albuminate of soda 



Lactates of potash and soda 



Sulphocyanide of potassium 



Soda 



Mucus, with some ptyalin 2 



1 ' Lorsqu'il n'y a point de parotides, comme cela a lieu dans Vechidne et le fourmi- 

 lier, la proportion des maxillaires augmente considerablement.' xu. vol. iv. p. 421. 



2 cxx". p. 417. 



