68 PHYSIOLOGY. 



of the lingual. The chorda tympani fibers run to the geniculate 

 ganglion and enter the brain with the fibers of the portio inter- 

 media of the seventh nerve. 



The afferent fibers of the glosso-pharyngeal go back to the 

 otic ganglion, then by the small superficial petrosal nerve and tym- 

 panic nerve to the nucleus of the glosso-pharyngeal. 



Trophic and Secretory Fibers. 



There are two kinds of fibers going to the salivary glands. If 

 the chorda be stimulated, it is found that the saliva contains more 

 water and salts in proportion to the organic matter than existed 

 before. If previous to the stimulation,, the gland was at rest and 

 not exhausted, the increase of the stimulation at first causes a rise 

 in the percentage of organic constituents, and this rise is more 

 notable than in the case of the salts. Hence Heidenhain held that 

 two kinds of nerve-fibers were distributed to the salivary glands. 

 One governs the secretion of water and salts, the other governs the 

 formation of the organic constituents of the saliva; the former he 

 called secretory fibers, the latter trophic. The sympathetic mainly 

 has trophic fibers, the chorda chiefly secretory fibers. 



Pawlow has shown in the dog that the submaxillary gland 

 reacts to a great number of stimuli, such as the sight of food 

 (psychical secretion), chewing of meats, and acids. The parotid 

 reacts only when dry food, dry bread or dry meat, is placed in the 

 mouth. Foods with a large amount of water excite a little flow of 

 saliva, whilst dry foods cause a more abundant flow. Here is an 

 adaptive capacity of the nerves of the salivary glands to the char- 

 acter of the food chewed. The reflex center for the salivary secre- 

 tion is situated in the medulla oblongata, near the origin of the 

 ninth and seventh cranial nerves. The afferent nerves are the 

 nerves of taste, the chorda tympani and the glosso-pharyngeal and 

 sensory branch of the trigeminus; the efferent nerves are the 

 auriculo-temporal and chorda tympani. 



GASTRIC DIGESTION (DIGESTION IN THE STOMACH). 



The stomach is the principal organ of digestion. As we know, 

 digestion has for its aim the rendition of the organic and inorganic 

 substances ingested from the external world into such a condition 

 that they can readily mix with the blood and so be introduced into 

 the living tissues of the body. For no animal can exist which does 

 not receive materials for its support from the environing media. 



