370 A MANUAL OF PHYSIOLOGY 



food is moist. Acids or salts induce an abundant flow, in order 

 that they may be neutralized, diluted or washed out of the 

 mouth. In this case a watery liquid, poor in mucin, flows from 

 the mucous glands. Mucin is a lubricant to facilitate the swal- 

 lowing of solid food, and here it could be of no use. When clean 

 pebbles are put in the dog's mouth the animal may try to chew 

 them, but eventually ejects them. Either no saliva or very 

 little is secreted, since it could not aid in their expulsion. If, 

 however, the very same stones are reduced to sand and again 

 introduced into the animal's mouth, saliva is plentifully secreted 

 to wash it out. 



The serous and mucous salivary glands are not necessarity 

 excited by the same food materials, and here again we can trace 

 an astonishingly exact adaptation. A permanent parotid or sub- 

 maxillary fistula can easily be made in a dog by freeing Stenson's 

 or Wharton's duct from the surrounding mucous membrane 

 for a little distance, bringing the natural orifice of the duct out 

 through a small wound in the cheek, and stitching it in position 

 there. When it is desired to collect saliva, the wide end of a 

 funnel-shaped tube, whose stem is bent so as to hang vertically, 

 can be attached by a little shellac of low melting-point to the 

 skin around the orifice of the duct and at some distance from it, 

 and on the narrow end can be hung a small graduated tube, into 

 which the saliva drops. When fresh meat is given to the animal 

 little or no parotid saliva is secreted, while a copious flow takes 

 place from the submaxillary gland, mucin being required to 

 lubricate it for deglutition, while water is not specially needed. 

 But if the meat is in the form of a dry powder the parotid pours 

 out a plentiful secretion, while the submaxillary also secretes a 

 fluid relatively rich in mucin. The same difference is seen 

 between fresh moist bread and dry bread. The afferent nerve- 

 endings from which impulses are carried to the reflex centres (or 

 the portions of the salivary centre) which preside over the various 

 salivary glands must possess the power of very delicate selection 

 as regards the kinds of stimulation by which they are affected. 

 The mere relish of the animal for the different kinds of food 

 plays but a small part. Most dogs display a much livelier 

 interest in a piece of meat than in a piece of dry biscuit, yet 

 it is the biscuit which excites the parotid to activity. 



The sight of dry food causes an abundant flow of watery saliva 

 from the parotid, and a flow of fluid rich in mucin from the sub- 

 maxillary. Various uneatable substances, including substances 

 which in contact with the mucous membrane of the mouth 

 produce strong and disagreeable stimulation of it, and excite dis- 

 gust, cause also, when viewed from a distance, secretion by all 

 the salivary glands ; but the submaxillary saliva, as ought to be 



