DIGESTION 371 



the case for substances unfit for food, and therefore not destined 

 to be swallowed, is poor in mucin. When the animal is shown 

 pebbles and sand the phenomena are qualitatively the same 

 as when they are put into its mouth the glands remaining 

 inactive in presence of the pebbles, but secreting plentifully at 

 sight of the sand. In short, the same adaptation is observed in 

 the case of the so-called psychical secretion as when the stimu- 

 lating substances act directly upon the endings of the afferent 

 salivary nerves in the buccal mucous membrane. It is further 

 worthy of note that when the animal is hungry the psychical 

 secretion is most copious and most easily obtained. After a 

 full meal it cannot be elicited at all. When food (or other 

 exciting substance) is repeatedly shown to a fasting animal the 

 reaction becomes each time weaker, and finally the glands 

 cease to respond. All that is then necessary to restore the re- 

 action is to put into the animal's mouth a little of the food (or 

 other object). When it is now sho\^n it at a distance the ordinary 

 effect follows promptly. This indicates that the condition of the 

 salivary centre exercises an important influence upon the psychical 

 secretion, its excitability to the weaker stimulus set up by the 

 sight of the object being increased by the stronger reflex stimula- 

 tion coming directly from the mouth. In the condition of 

 satiety the inexcitability of the centre may be due to the action 

 of food-products in the blood. 



In most animals and in man the activity of the large salivary 

 glands is strictly intermittent. But the smaller glands that stud 

 the mucous membrane of the mouth never entirely cease to secrete, 

 and the same is the case with the parotid in ruminant animals. 



The centre is situated in the medulla oblongata, stimulation 

 of which causes a flow of saliva. The chief afferent paths to the 

 salivary centre are the lingual branch of the fifth and the glosso- 

 pharyngeal ; but stimulation of many other nerves may cause 

 reflex secretion of saliva. In experimental reflex stimulation, the 

 sole efferent channel seems to be the cerebral nerve-supply of the 

 glands. After section of the chorda, no reflex secretion by the 

 submaxillary gland can be caused, although the sympathetic 

 remains intact. 



It was alleged by Bernard that, after division of the chordo- 

 lingual, a reflex secretion could be obtained from the submaxillary 

 gland by stimulating the central end of the cut lingual nerve 

 between the so-called submaxillary ganglion and the tongue, the 

 ganglion being supposed to act as ' centre/ It has been shown, 

 however, that this is not a true reflex effect, but is due to the 

 excitation of certain (recurrent) secretory fibres of the chorda 

 that run for some distance in the lingual, then bend back on 

 their course and pass to the gland. It may be in part a pseudo- 



24 2 



