B38 SECRETION OF GASTRIC JUICE. [BOOK ir. 



gland is almost arrested. The sympathetic therefore acts as a 

 vaso-constrictor nerve, and in this sense is antagonistic to the 

 chorda. 



As concerns the flow of saliva brought about by stimulation of 

 the sympathetic, in the case of the submaxillary gland of the dog 

 the effects are very peculiar. A slight flow results, and the saliva 

 so secreted is remarkably viscid, of higher specific gravity, and 

 richer in corpuscles and in the above-mentioned amorphous lumps 

 than is the chorda saliva. This action of the sympathetic is little 

 or not at all affected by atropin. 



In the submaxillary gland of the dog then the contrast between 

 the effects of chorda stimulation and those of sympathetic stimu- 

 lation are very marked : the former gives rise to vascular dilation 

 with a copious flow of fairly limpid saliva poor in solids, the latter 

 to vascular constriction with a scanty flow of viscid saliva richer 

 in solids. And in other animals a similar contrast prevails, though 

 with minor differences. We shall return again presently to these 

 different actions of the two nerves ; meanwhile we have seen 

 enough of the history of the submaxillary gland to learn that 

 secretion in this instance is a reflex action, the efferent impulses 

 of which directly affect the secreting cells, and that the vas- 

 cular phenomena may assist, but are not the direct cause of, the 

 flow. 



191. We have dwelt long on this gland because it has 

 been more fruitfully studied than any other But the nervous 

 mechanisms of the other salivary glands are in their main features 

 similar. Thus the secretion of the parotid gland, like that of the 

 submaxillary, is governed by two sets of fibres : one of cerebro- 

 spinal origin, running along the auriculo-temporal branch of 

 the fifth nerve but originating possibly in the glossopharyngeal, 

 and the other of sympathetic origin coming from the cervical 

 sympathetic. Stimulation of the cerebro-spinal fibres produces a 

 copious flow of limpid saliva, free from mucus ; stimulation of the 

 cervical sympathetic gives rise in the rabbit to a secretion also free 

 from mucus but rich in proteids and of greater amylolytic power 

 than the cerebro-spinal secretion ; in the dog little or no secretion 

 is produced, though, as we shall see later on, certain changes are 

 brought about in the gland itself. In both animals the cerebro- 

 spinal fibres are vaso-dilator, and the sympathetic fibres vaso- 

 constrictor in action. 



192. The secretion of gastric juice. Though a certain amount 

 of gastric juice may sometimes be found in the stomachs of fasting 

 animals, it may be stated generally that the stomach, like the 

 salivary glands, remains inactive, yielding no secretion, so long as 

 it is not stimulated by food or otherwise. The advent of food into 

 the stomach however at once causes a copious flow of gastric juice ; 

 and the quantity secreted in the twenty-four hours is probably very 

 considerable, but we have no trustworthy data for calculating the 



