238 



THE MECHANISM OF LIFE 



Two nerves enter the gland. One of these comes from the 

 medulla, and is called the chorda tympani. The other comes 

 from the sympathetic nervous system. These are efferent nerves 

 conveying stimuli to the gland from the central nervous system. 



The duct divides into (or, rather, is formed by the union of) a 

 great number of smaller ductules or tubes. The ends of these 

 tubes may be thought about as swelling out to form bulbar 

 enlargements. The walls of the ductules are thin, but those of 

 the enlargements, or alveoli, are thick, being made up of large 

 cells. It is in these cells that the actual formation of saliva 

 occurs. 



The artery which enters the gland divides up into a great 

 number of smaller vessels, or arterioles, and the latter again 



B 



Fig. 53. 



A, Diagrammatic section through an alveolus (or secreting unit) of the 

 gland, with its capillary circulation; B, diagram of the terminations 

 of a secretory nerve among the cells of an alveolus. 



break up into capillaries. The capillaries are distributed over 

 the alveoli, as shown in Fig. 53. After circulating through the 

 capillaries, the blood leaves the gland by means of the vein. 



The nerves similarly break up into smaller branches. Some 

 of these divide further into very fine fibrils, which are distributed 

 as a fine network over the alveoli on the outside, or even in 

 between, the cells. Other branches of both nerves break up into 

 fibres which end in the muscles of the walls of the artery and 

 arterioles. 



Such is the structure. How does it function ? 



Generally saliva flows into the mouth when savoury food 

 substances stimulate the gland to act. This is a reflex, and the 

 afferent nerves are the gustatory and olfactory ones — ^that is, 

 those conveying the taste and smell stimuli to the brain. But 

 this is not all by any means, for if meat be shown to a hungry 



