1 62 A MANUAL OF VETERINARY PHYSIOLOGY 



albuminous) gland, and this holds good in all animals ; but the 

 submaxillary is sometimes of the mucous type, as in the dog 

 and cat, and sometimes mixed, as in man. The sublingual may 

 also be a mixed gland, though with mucous cells predominating. 



Mucous glands are generally characterised by the presence of 

 peculiar crescent-shaped cells known as demilunes, lying beneath 

 the basement membrane, and away from the central lumen 

 of the tube. Great difference of opinion has existed as to the 

 special function of these crescents of Gianuzzi, some believing that 

 they replace worn-out mucous cells, others that they possess 

 specific functions. 



It has been supposed that the activity of the three glands 

 depends upon the character of the food substances, and in the 

 dog there is experimental proof of the correctness of this view. 

 Pawlow, whose work on digestion has opened up a new field, 

 has shown that in the dog — and the following remarks refer solely 

 to this animal — the submaxillary gland responds to the sight of 

 food, to the chewing of meat and the action of acids ; while 

 the parotid responds to dry food, such as dry powdered meat, 

 bread, or biscuit. 



The selective power of the glands and their adaptability to 

 the class of food in the mouth is very remarkable. Dry bread 

 excites the parotid, since water is required to moisten it, and the 

 submaxillary since mucin is needed in order to lubricate it, but 

 moist bread only stimulates the submaxillary. Fresh meat 

 requires no parotid, but only submaxillary saliva. Pebbles 

 placed in the mouth excite little or no secretion, but pebbles 

 reduced to sand excite an abundant secretion. It has been 

 suggested that in this experiment the abundant secretion is to 

 wash out the sand, while pebbles require no washing out, as 

 they can be dropped. Still more remarkable, it has been shown 

 that the sight of dry food causes an abundant secretion of watery 

 saliva and a flow of rich, mucinous saliva. 



From this it is evident that no stimulation of the buccal 

 mucous membrane is essential to secretion, though there can be 

 little doubt that the adaptative mechanism is frequently pro- 

 voked through this channel. 



In the herbivora Colin was able to show that the secretion of 

 saliva was uninfluenced by the sight or smell of food ; nor could 

 he obtain any secretion from the parotids by the employment 

 of excitants to the mucous membrane of the mouth ; salts, acids, 

 aromatic substances, were all equally negative in this respect. 

 The submaxillary and sublingual glands, on the other hand, 

 actively responded to these stimuli. In the horse he found 

 that oats produced a greater secretion of saliva than hay, though 

 the amount of fluid absorbed by oats is one-quarter that of hay. 



