INFLUENCE OF NERVOUS SYSTEM ON DIGESTIVE GLANDS 399 



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 necessarily excited 

 by the same food materials, and here again we can trace an astonish- 

 ingly exact adaptation. A permanent parotid or submaxillary 

 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 disgust, cause 

 also, when viewed from a distance, secretion by all the salivary 

 glands; but the submaxillary saliva, as ought to be the case for 

 substances unfit for food, and therefore not destined to be swallowed, 

 is poor in mucin. When the animal is shown pebbles and sand 

 the phenomena are qualitatively the same as when they are put 

 into its mouth the glands remaining inactive in presence of 

 the pebbles, but secreting plentifully at sight of the sar^d. In 

 short, the same adaptation is observed in the case of the so-called 

 psychical secretion as when the stimulating substances act directly 



