398 MATER i A MEDIC A AND THERAPEUTICS. 



salts, and water. The relative proportions of these constituents 

 vary greatly in different kinds of food. 



2. The sensory nerves of the mouth (the glosso-pharyngeal, 

 and the lingual and other branches of the trigeminus) receive 

 and transmit to the cerebrum and medulla the impressions of 

 taste, as they are commonly called, whether sweet (the pleasant 

 taste referable toamylolytic action), bitter, salt, sour, hot, burn- 

 ing, warm, pungent, acrid, or nauseous ; and the many kinds of 

 aromatic flavours, which are chiefly, however, odours. In the 

 medulla the gustatory impressions fall into a special centre, 

 whence they are reflected (1) to the stomach, the functions of 

 which they modify, as we shall see ; and (2) to the salivary 

 and mucous glands of the mouth, which they also influence, 

 chiefly through the chorda tympani. Through the same efferent 

 nerve come other impulses : from the cerebrum, as the result of 

 the sight, taste, smell, or even idea of food ; from the stomach, 

 conveyed by the vagus ; and, doubtless, from many other sensi- 

 tive parts, especially in the abdomen. 



3. The flow of saliva and mucus is the result of the nervous 

 impulses which have just been traced, and which stimulate 

 the protoplasm of the epithelial cells, and actively dilate 

 the vessels. The saliva is secreted at the commencement of 

 digestion, is intimately mixed with the food, and imparts to 

 the bolus a faintly alkaline reaction which has an important 

 effect on secretion in the stomach. 



4. It is well to distinguish from the ordinary secretions of the 

 mouth, the excretions which are also thrown out by the glands. 

 Although these are but little appreciated in health, they are 

 familiar as the source of certain unpleasant tastes in the mouth 

 and odours of the breath, after particular kinds of food and 

 drink, such as wines, and many drugs. 



5. The muscular acts of mastication and swallowing are 

 guided by the afferent impressions and by the will. 



II. PHARMACODYNAMICS. 



We come now to inquire, according to the plan which we 

 have sketched, whether we possess any means of influencing 

 the normal functions of the mouth, and if so, how far such 

 powers can be usefully applied. 



1. Food. We have absolute control over our food. We can 

 withhold it altogether ; we can alter its quantity and its 

 quality as we please. Especially as regards the mouth, we may 

 modify the proportion of amyloids in the diet, affect their con- 

 dition by cookery, or convert them wholly or partially into 

 sugar before administration. Malt extracts consist chiefly 

 of dextrin and maltose, made from malted grain and flour. 



