SALIVA. 127 



saliva contains sulpho-cyanogen, which may be detected, as we have 

 already seen, after the organic matters have been precipitated by 

 alcohol. 



A very curious fact has been observed by M. Colin, Professor of 

 Anatomy and Physiology at the Veterinary School of Alfort, 1 viz., 

 that in the horse and ass, as well as in the cow and other ruminat- 

 ing animals, the parotid glands of the two opposite sides, during 

 mastication, are never in active secretion at the same time ; but 

 that they alternate with each other, one remaining quiescent while 

 the other is active, and vice versa. In these animals mastication is 

 said to be unilateral, that is, when the animal commences feeding 

 or ruminating, the food is triturated, for fifteen minutes or more, by 

 the molars of one side only. It is then changed to the opposite 

 side ; and for the next fifteen minutes mastication is performed by 

 the molars of that side only. It is then changed back again, and 

 so on alternately, so that the direction of the lateral movements of 

 the jaw may be reversed many times during the course of a meal. 

 By establishing a salivary fistula simultaneously on each side, it is 

 found that the flow of saliva corresponds with the direction of the 

 masticatory movement. \Yhen the animal masticates on the right 

 side, it is the right parotid which secretes actively, while but little 

 saliva is supplied by the left ; when mastication is on the left side, 

 the left parotid pours out an abundance of fluid, while the right is 

 nearly inactive. 



We have observed a similar alternation in the flow of parotid 

 saliva in the human subject, when the mastication is changed from 

 side to side. In ah experiment of this kind, the tube being inserted 

 into the parotid duct of the left side, the quantity of saliva dis- 

 charged during twenty minutes, while mastication was performed 

 mainly on the opposite side of the mouth, was 127.5 grains ; while 

 the quantity during the same period, mastication being on the same 

 side of the mouth, was 374.4 grains being nearly three times as 

 much in the latter case as in the former. 



Owing to the variations in the rapidity of its secretion, and also 

 to the fact that it is not so readily excited by artificial means as 

 by the presence of food, it becomes somewhat difficult to estimate 

 the total quantity of saliva secreted daily. The first attempt to do so 

 was made by Mitscherlich, 2 who collected from two to three ounces 

 in twenty-four hours from an accidental salivary fistula of Steno's 



1 Traite de Physiologie Comparee, Paris, 1854, p. 468. 

 1 Simon's Chemistry of Mail. Phila. ed., 1846, p. 295. 



