392 DIGESTION 



corresponding side, ana is therefore due to impulses arising in the 

 central nervous system. The cause of the paralytic secretion has not 

 been fully made out. If within two or three days of division of the 

 chorda the sympathetic on the same side is cut, the secretion is greatly 

 diminished or stops altogether; and it is concluded that up to this time 

 it is maintained by impulses passing along the sympathetic to the gland 

 from the salivary centre, the excitability of which has been in some way 

 increased by division of the chorda, possibly by some such degenerative 

 process in the cells as* the changes seen in cerebro-spinal motor cells 

 whose axons have been divided (p. 830). This may also account for the 

 antilytic secretion. But if section of the sympathetic is not performed 

 for several days, it has no effect on the paralytic secretion, which at 

 this stage seems to depend on local changes in or near the gland itself, 

 leading to a mild continuous excitation of those nerve-cells on the 

 course of the fibres of the chorda to which reference has already been 

 made. Section of the sympathetic alone causes neither secretion nor 

 atrophy, nor does removal of the superior cervical ganglion. The 

 histological characters of the gland-cells during paralytic secretion are 

 those of ' rest.' 



Reflex Secretion of Saliva. The reflex mechanism of salivary 

 secretion is very mobile, and easily set in action by physical and 

 mental influences. It is excited normally by impulses which arise 

 in the mouth, especially by the contact of food with the buccal 

 mucous membrane and the gustatory nerve-endings. The mere 

 mechanical movement of the jaws, even when there is nothing 

 between the teeth, or only a bit of a non-sapid substance like india- 

 rubber, causes some secretion. The vapour of ether gives rise to a 

 rush of saliva, as does gargling the mouth with distilled water. 

 The smell, sight, or thought of food, and even the thought of saliva 

 itself, may act on the salivary centre through its connections with 

 the cerebrum, and make ' the teeth water.' A copious flow of 

 saliva, reflexly excited through the gastric branches of the vagus, 

 is a common precursor of vomiting. The introduction of food into 

 the stomach also excites salivary secretion. 



The researches of Pawlow and his pupils have shown that the 

 salivary glands are not excited indifferently by everything which 

 comes into contact with the buccal mucous membrane. A remark- 

 able adaptation exists between the properties of food or foreign 

 bodies introduced into the mouth and their effects upon the secre- 

 tion of saliva. When solid dry food is given to a dog saliva is 

 copiously poured out; much less is secreted when the food is moist. 



Acids or salts induce an abundant flow, in order that they may 

 be neutralized, diluted or washed out of the mouth. In this case 

 a watery liquid, poor in mucin, flows from the mucous glands. 

 Mucin is a lubricant to facilitate the swallowing of solid food, and 

 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 



