DIGESTION. 237 



more solids than either of the other two, but does not appear to be so 

 powerful in its action. 



The salivary glands of children do not become functionally active till 

 the age of 4 to 6 months, and hence the bad effect of feeding them be- 

 fore this age on starchy food, corn-flour, etc., which they are unable to 

 render soluble and capable of absorption. 



Influence of the Nervous System. 



The secretion of saliva is under the control of the nervous system. 

 It is a reflex action. Under ordinary conditions it is excited by the 

 stimulation of the peripheral branches of two nerves, viz., the gustatory 

 or lingual branch of the inferior maxillary division of the fifth nerve, 

 and the glosso-pharyngeal part of the eighth pair of nerves, which are 

 distributed to the mucous membrane of the tongue and pharynx con- 

 jointly. The stimulation occurs on the introduction of sapid substances 

 into the mouth, and the secretion is brought about in the following way. 

 From the terminations of the above-mentioned sensory nerves distributed 

 in the mucous membrane an impression is conveyed upwards (afferent) 

 to the special nerve-centre situated in the medulla, which controls the 

 process, and by it is reflected to certain nerves supplied to the salivary 

 glands, which will be presently indicated. In other words, the centre, 

 stimulated to action by the sensory impressions carried to it, sends out 

 impulses along efferent or secretory nerves supplied to the salivary glands, 

 which cause the saliva to be secreted by and discharged from the gland 

 cells. Other stimuli, however, besides that of the food, and other sen- 

 sory nerves besides those mentioned, may produce reflexly the same ef- 

 fects. For example, saliva may be caused to flow by irritation of the 

 mucous membrane of the mouth with mechanical, chemical, electrical, 

 or thermal stimuli, also by the irritation of the mucous membrane of 

 the stomach in some way, as in nausea, which precedes vomiting, when 

 some of the peripheral fibres of the vagi are irritated. Stimulation of 

 the olfactory nerves by smell of food, of the optic nerves by the sight of 

 it, and of the auditory nerves by the sounds which are known by expe- 

 rience to accompany the preparation of a meal, may also, in the hungry, 

 stimulate the nerve-centre to action. In addition to these, as a secretion 

 of saliva follows the movement of the muscles of mastication, it may be 

 assumed that this movement stimulates the secreting nerve-fibres of the 

 gland, directly or reflexly. From the fact that the flow of saliva may be 

 increased or diminished by mental emotions, it is evident that impres- 

 sions from the cerebrum also are capable of stimulating the centre to ac- 

 tion or of inhibiting its action. 



Salivary secretion may also be excited by direct stimulation of the 

 centre in the medulla. 



