CHAP, i.] DIGESTION. 277 



the submaxillary gland of the dog, stimulation of the chorda 

 tympani produces a copious flow of thin limpid saliva, and stimu- 

 lation of the cervical sympathetic a scanty flow of thick viscid 

 saliva. That is to say, stimulation of the chorda affects chiefly the 

 discharge of water, which carries away with it various soluble 

 matters, while stimulation of the sympathetic chiefly affects the 

 conversion of mucigen into mucin. To this we may add the case 

 of the parotid of the dog. In this stimulation of a cerebro-spinal 

 nerve, the auriculo-temporal, produces a copious flow of limpid 

 saliva, while stimulation of sympathetic produces itself little or no 

 secretion at all ; but after previous stimulation of the sympathetic, 

 the saliva which flows upon stimulation of the cerebro-spinal nerve 

 is much richer in solid and especially in organic matter. And we 

 have already seen that while the microscopic changes after cerebro- 

 spinal stimulation are inappreciable, those following upon sym- 

 pathetic stimulation are very conspicuous. The latter gives rise to 

 certain constituents, while the former, so to speak, washes them 

 away into the duct. 



These and other facts, on which we need not now dwell, have 

 led to the conception that the act of secretion consists of two 

 parts, both distinct efforts of the cell, which in one case may 

 coincide, in another may take place apart or in different pro- 

 portions. On the one hand, there is the discharge of water 

 carrying with it common soluble substances; on the other, the 

 escape of specific substances resulting from the profound meta- 

 bolism of the cell protoplasm. And it has been supposed that 

 two kinds of nerve fibres exist, one governing the former process 

 and preponderating in the chorda tympani, for instance, the other 

 governing the latter and preponderating in the branches of the 

 cervical sympathetic. Further hypotheses have been put forward 

 to explain the modus operandi of the discharge of water, such 

 as the existence of substances in the cell which absorb water from 

 the blood or lymph on the one side and give it up on the other 

 side into the lumen of the alveolus. But these matters are not 

 yet ripe for any distinct assertion, and though we have thought it 

 right to bring the matter before our readers, we must not pursue 

 the discussion any further. Whether there be two sets of fibres or 

 no, whether the two processes be absolutely distinct or merely 

 variations of the same fundamental changes, the proposition on 

 which we have so long dwelt that the flow of juice from a 

 secreting gland is essentially the outcome of the activity of the 

 secreting cell remains equally true. 



Before we leave the mechanism of secretion there are one 

 or more accessory points which deserve attention. 



In treating just now of the gastric glands we spoke as if 

 pepsin were the only important constituent of gastric juice, 

 whereas, as we have previously seen, the acid is equally essential. 

 The formation of the free acid of the gastric juice is very 



