276 THE TISSUES AND MECHANISMS OF DIGESTION. 



and especially in organic matter than when the cerebro-spinal nerve is stimu- 

 lated alone. And we have already seen that in this gland the microscopic 

 changes following upon sympathetic stimulation are more conspicuous than 

 those which follow upon cerebro-spinal stimulation. 



These and other facts have led to the conception that the act of secretion 

 consists of two parts, which in one case may coincide, in another may take 

 place apart or in different proportions. On the one hand, there is the dis- 

 charge of water carrying with it common soluble substances, chiefly salines, 

 derived from the blood ; on the other haud, a metabolic activity of the cell- 

 substance gives rise to the specific constituents of the juice. To put the 

 matter broadly, the latter process produces the specific constituents, the 

 former washes these and other matters into the duct. It has been further 

 supposed that two kinds of nerve fibres exist : one governing the former 

 process and, in the case of the submaxillary gland for instance, prepon- 

 derating, though not to the total exclusion of the other kind, in the chorda 

 tympani ; the other governing the latter process and preponderating in the 

 branches of the cervical sympathetic. These have been called respectively 

 " secretory " and " trophic " fibres ; but these terms are not desirable. It 

 may here be remarked that even the former process is a distinct activity of 

 the gland and not a mere infiltration. For, as we have seen in the case of 

 the salivary glands, when atropine is given, not only do the specific constitu- 

 ents cease to be ejected as a consequence of stimulation of the chorda, but 

 the discharge of water, in spite of the bloodvessels becoming dilated, is also 

 arrested : no saliva at all leaves the gland. And what is true of the salivary 

 glands as regards the dependence of the flow of water on something else 

 besides the mere pressure of the blood in the bloodvessels appears to hold 

 good with other glands also. Indeed, it has been suggested that the very 

 discharge of water is due to an activity of the cell ; the hypothesis has been 

 put forward that changes in the cell give rise to the formation in the cell of 

 substances 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. Such an 

 hypothesis cannot be regarded as proved ; but the mere putting it forward 

 raises a doubt as to the validity of the distinction on which we have been 

 dwelling; and other considerations point in the same direction. For in- 

 stance, if the common soluble salts present in a juice, as distinguished from 

 the specific constituents, were merely carried into the juice by the rush, so 

 to speak, of water, we should expect to find the percentage of these salts 

 either remaining the same or perhaps decreasing when the juice was secreted 

 more rapidly and in fuller volume. But under these circumstances the per- 

 centage very frequently increases ; and in general we find that under various 

 circumstances the proportion of salts secreted to the quantity of water 

 secreted may vary considerably. Obviously, while something determines 

 the quantity of water passing into the alveolus, something else determines 

 how much of common soluble salts that water contains, and still something 

 else determines to what extent that water is also laden with specific constitu- 

 ents and other organic bodies. The whole action is too complicated to be 

 described as consisting merely of the two processes mentioned above, but the 

 time has not yet come for clear and definite statements. Everything, how- 

 ever, tends to show that the cell is the prime agent in the whole business, 

 though we cannot at present define the nature of the several changes in the 

 cell, nor can we say how those changes are exactly related to each other, 

 to changes of the blood-pressure in the bloodvessels, or, we may add, to 

 changes taking place in the lymph-spaces which lie between the blood and 

 the cell. 



We may perhaps add that since in certain cutaneous secreting glands 



