CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 425 



As concerns the effects brought about by stimulation of the 

 sympathetic nerve, these, in the case of the submaxillary gland of 

 the dog, 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. The fibres carrying this influence 

 may, like the vaso-constrictor impulses, be traced back along the 

 cervical sympathetic to the spinal cord. 



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, 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. Thus in the rabbit both chorda saliva and 

 sympathetic saliva are limpid and free from mucus, though the 

 latter contains more proteids; in the cat, chorda saliva is more 

 viscid than sympathetic saliva; but in both these cases, as in 

 the dog, stimulation of the chorda causes a copious flow with 

 dilated blood vessels, and stimulation of the sympathetic a scanty 

 flow with vascular constriction. 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 vascular 

 phenomena may assist, but are not the direct cause of, the flow. 



229. 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. Stimulation of the central end of 

 the glossopharyngeal produces by reflex action a secretion from the 

 parotid gland, but that of the lingual is said to be without 

 effect. 



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

 F. ii. 28 



OF THE 



