REFLEX SECRETION OF SALIVA. 491 



tinuous parotid secretion of ruminants ; but do nevertheless cause a secretion 

 from the submaxillary and sublingual glands. His statements, however, are not 

 quite consistent, and we may suppose that the difference is only one of degree. 



In herbivora, mastication is performed alternately on the two sides, 

 the periods being usually one quarter to half an hour. In the horse, and 

 probably in other herbivorous animals, the secretion from the parotid is 

 much greater on the masticating than on the non-masticating side. In 

 the horse two to three times as much saliva is usually secreted on the 

 masticating as on the opposite side, but the ratio may be either greater 

 or less than this (Bernard, Colin). It seems reasonable to suppose that 

 this is due to the better contact of food with the mucous membrane of 

 that side of the mouth. According to Colin, however, there is no such 

 difference in the secretion of the submaxillary and sublingual glands. 



The amount of saliva secreted varies with the nature of the food. 

 In man the data are not sufficient to form an estimate of any value, 

 either of the relative amount of saliva secreted from the several glands, 

 or of the total amount secreted in twenty-four hours. It is generally 

 supposed that the total amount exceeds a litre a day. 1 



Bernard 2 found that in the clog, when saliva was obtained reflexly, 

 the submaxillary gland secreted about twice as much as the parotid, and 

 about ten times as much as the sublingual. And these are approxi- 

 mately the relative amounts obtained by injecting pilocarpine. The 

 amounts secreted are roughly in proportion to the respective weights of 

 the glands. 



In herbivora the volume of saliva secreted by the submaxillary and 

 sublingual glands does not correspond with their respective weights. 

 According to Colin, the parotid gland of the horse secretes fifteen to 

 twenty times the volume of saliva secreted by the submaxillary gland, 

 but is only about four times its weight. And in the ox the parotid 

 secretes four to five times as much saliva as the submaxillary gland, 

 though it is slightly less in weight (cf. Table, p. 477). 



Colin estimates that in the horse the total quantity of saliva secreted 

 in a day is about 40 litres. 



The total quantity of saliva may be estimated in two ways (1) By com- 

 paring the weights of a certain amount of food before and after mastication 

 and swallowing, the food after mastication being collected from an ossophageal 

 fistula ; and (2) by noting in different experiments the quantity secreted by 

 each gland during a given period of feeding. Colin found, by the first method, 

 that a horse secreted 5000 to 6000 grins, of saliva in an hour, when fed with 

 hay, one-half of this when fed with grass, and one-third more than this when 

 fed with oats. 



During digestion, according to Colin, the submaxillary gland of one side 

 secretes 25 to 30 c.c. of saliva in fifteen minutes; the parotid secretes 500 to 

 1000 (about) in fifteen minutes, if mastication takes place on this side. 3 



In the ox he estimates that during three hours' mastication, and five 

 hours' rumination, about 40 litres of saliva are secreted, and that 16 litres are 

 secreted during the sixteen hours of rest. 



Electrical excitation of the central end of the lingual or of the glosso- 

 pharyngeal causes secretion from all the salivary glands. The secretion 



1 Some data are given by Tuczek, Ztschr. f. BioL, Miinchen, 1876, Bd. xii. S. 534. 



" Lecons de physiol. exper.," 1856, tome ii. p. 82. 

 3 For other observations on this point, cf. Elleaberger and Hofmeister, op. cit., supra. 



