CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 401 



fibres may be traced into the cells. But, save perhaps in the case of 

 certain glands of invertebrates (so-called salivary glands of Blatta), 

 the evidence MS we have said is as yet not convincing. 



228. When the cervical sympathetic is stimulated, the \ 

 vascular effects, as we have already said, 1G8, are the exact I 

 contrary of those seen when the chorda is stimulated. The small 

 arteries are constricted, and a small quantity of dark venous blood 

 escapes by the veins. Sometimes, indeed, the flow through the 

 gland is almost arrested. The sympathetic therefore acts as a 

 vasoconstrictor 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. 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 

 the fifth nerve but originating possibly in the glossopharyugeal, 

 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, the secretion 

 reaching in the dog a pressure of 118 mm. mercury; stimulation 

 of the cervical sympathetic gives rise in the rabbit to a secretion 



F. 26 



