CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 351 



promoting the discharge of water, while stimulation of the sym- 

 pathetic has a marked effect in promoting the discharge of niucin. 

 To this we may add the case of the parotid of the dog. In this 

 gland stimulation of a cerebro-spinal nerve, the auriculo-temporal, 

 produces a copious flow of limpid saliva, while stimulation of the 

 sympathetic produces itself little or no secretion at all ; but when 

 the sympathetic and cerebro-spinal nerves are stimulated at the 

 same time, the saliva which flows is much richer in solid and 

 especially in organic matter than when the cerebro-spinal nerve 

 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. The whole act of secretion is a very complicated 

 one, probably too complicated to be described as consisting merely 

 of the two processes mentioned above. 



202. Throughout the above we have spoken as if the secre- 

 tion were furnished exclusively by the cells of the alveoli or se- 

 creting portion of the gland, as if the epithelium cells lining the 

 ducts, or conducting portion of the gland, contributed nothing to 

 the act. In the gastric glands, the slender cells lining the mouths 

 of the glands (which correspond to ducts) and covering the ridge? 



