CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 417 



is stimulated 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 propor- 

 tions. On the one hand, there is the discharge of water carrying 

 with it common soluble substances, chiefly salines, derived from 

 the blood ; on the other hand, 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, preponderating, 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 be here remarked that even the former 

 process is a distinct activity of the gland, and not a mere filtra- 

 tion. For, as we have seen in the case of the salivary glands, 

 when atropin is given, not only do the specific constituents cease 

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

 the discharge of water, in spite of the blood vessels 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 blood vessels, 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 doubts as to the validity 

 of the distinction on which we have been dwelling ; and other 

 considerations point in the same direction. For instance, 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 percentage 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 



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