SECRETION. 165 



constituents or cause only a temporary increase, quickly followed by a fall. 

 Similar results may be obtained from stimulation of the cerebral nerves of 

 the parotid gland. The above facts led Heidenhain to believe that the con- 

 ditions determining the secretion of the organic material are different from 

 those controlling the water and salts, and he gave a rational explanation of 

 the differences observed, in his theory of trophic and secretory fibres. 



Theory of Trophic and Secretory Nerve-fibres. This theory supposes 

 that two physiological varieties of nerve-fibres are distributed to the salivary 

 glands. One of these varieties controls the secretion of the water and inor- 

 ganic salts and its fibres may be called secretory fibres proper, while the other, 

 to which the name trophic is given, causes the formation of the organic con- 

 stituents of the secretion, probably by a direct influence on the metabolism in 

 the cell. Were the trophic fibres to act alone, the organic products would be 

 formed within the cell but there would be no visible secretion, and this is 

 the hypothesis which Heidenhain uses to explain the results of the experi- 

 ment described above upon stimulation of the sympathetic fibres to the parotid 

 of the dog. In this animal, apparently, the sympathetic branches to the parotid 

 contain exclusively or almost exclusively trophic fibres, while in the cerebral 

 branches both trophic and secretory fibres proper are present. The results of 

 stimulation of the cerebral and sympathetic branches to the submaxillary gland 

 of the same animal may be explained in terms of this theory by supposing that 

 in the latter nerve trophic fibres preponderate, and in the former the secretory 

 fibres proper. 



It is obvious that this anatomical separation of the two sets of fibres along the 

 cerebral and sympathetic paths may be open to individual variations, and that 

 dogs may be found in which the sympathetic branches to the parotid glands 

 contain secretory fibres proper, and therefore give some flow of secretion on 

 stimulation. These variations might also be expected to be more marked when 

 animals of different groups are compared. Thus Langley 1 finds that in cats the 

 sympathetic saliva from the submaxillary gland is less viscid than the chorda 

 saliva, just the reverse of what occurs in the dog. To apply Heidenhain's 

 theory to this case it is necessary to assume that in the cat the trophic fibres 

 run chiefly in the chorda. An interesting fact with reference to the secretion 

 of the parotid in dogs has been noted by Langley and is of special interest, 

 since, although it may be reconciled with the theory of trophic and secretory 

 fibres, it is at the same time suggestive of an incompleteness in this theory. 

 As has been said, stimulation of the sympathetic in the clog causes usually no 

 secretion from the parotid. Langley 2 finds, however, that if the tympanic 

 nerve is stimulated just previously, stimulation of the sympathetic causes 

 a secretory flow from the parotid. One may explain this in terms of the 

 theory by assuming that the sympathetic does contain a few secretory fibres 

 proper, but that ordinarily their action is too feeble to start the flow of water. 

 Previous stimulation of the tympanic nerve, however, leaves the gland-cells in 



1 Journal of Physiology, 1878, vol. i. p. 96. 



2 Ibid., 1889, vol. x. p. 291. 



