306 VETERINARY PHYSIOLOGY 



directly by the condition of the blood. Stimulation of the 

 lingual or glossopharyngeal leads to a reflex flow of saliva. 

 Other nerves may also act on this centre. Thus gastric 

 irritation, when it produces vomiting, causes a reflex stimu- 

 lation of salivary secretion. In asphyxia the condition of 

 the blood may directly stimulate the centre. 



According to the investigations of Pavlov, the salivary 

 glands react appropriately to different kinds of stimuli 

 through their nervous mechanism. When sand or bitter 

 or saline substances are put in a dog's moutb, a profuse 

 secretion of very watery saliva ensues to wasli them out. 

 When flesh is given, a saliva rich in mucin is produced. 

 When dry food is given, saliva is produced in greater 

 quantity than when moist food is supplied. 



Pavlov further states that the sight of diff'erent kinds of 

 food produces a flow of the kind of saliva which their 

 presence in the mouth would produce. The flow of saliva 

 has been used by him for the study of cerebral, or 

 "conditioned," reflexes. 



II. SWALLOWING. 



1. Voluntary Stage, — The food, after being masticated, is 

 collected on the surface of the tongue by the voluntary 

 action of the buccinators and other muscles, and then, the 

 point of the tongue is pressed against the hard palate behind 

 the teeth. By a contraction passing backwards the parts of 

 the tongue behind the tip are raised, along with the hyoid 

 and with tlie larynx, and tlie bolus of food is thus pushed 

 along the palate and through the pillars of the fauces. 



2. Reflex Stage. — Swallowing now becomes a pure reflex. 

 It can be performed only if something is present to excite it, 

 and it is abolished by paralysing the receptors in the pharynx 

 by means of cocaine. 



(1) The exact position of the receptor spots, stimulation 

 of which thus reflexly causes swallowing, varies in ditferent 

 animals. In man they are chiefly about the base of the 

 tongue and the posterior pharyngeal wall. Supplementary 



