672 PHYSIOLOGY OF DIGESTION AND SECRETION. 



gland which had previously been secreting actively. With regard 

 to the organic constituents the precise result obtained depends on 

 the condition of the gland. If previous to the stimulation the gland 

 was in a resting condition and unfatigued, then increased strength 

 of stimulation is followed at first by a rise in the percentage of organic 

 constituents, and this rise in the beginning is more marked than in 

 the case of the salts. But with continued stimulation the increase 

 in organic material soon ceases, and finally the amount begins actually 

 to diminish, and may fall to a low point in spite of the stronger 

 stimulation. On the other hand, if the gland at the beginning of the 

 experiment had been previously worked to a considerable extent, 

 then an increase in the stimulating current, while it augments the 

 amount of water and salts, either may have no effect at all upon the 

 organic constituents or may 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 conditions 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 fibers. 



Theory of Trophic and Secretory Nerve Fibers. This theory 

 supposes that two physiological varieties of nerve fibers are distrib- 

 uted to the salivary glands. One of these varieties controls the 

 secretion of the water and inorganic salts and its fibers may be called 

 secretory fibers proper, while the other, to which the name trophic 

 is given, causes the formation of the organic constituents of the secre- 

 tion, probably by a direct influence on the metabolism of the cells. 

 Were the trophic fibers 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 experiment described above upon stimulation of the sympathetic 

 fibers to the parotid of the dog. In this animal, apparently, the 

 sympathetic branches to the parotid contain exclusively or almost 

 exclusively trophic fibers, while in the cerebral branches both trophic 

 and secretory fibers 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 fibers preponderate, and 

 in the former the secretory fibers proper. 



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

 fibers along the cerebral and sympathetic paths may be open to 

 individual variations, and that dogs may be found in which the sym- 

 pathetic branches to the parotid glands contain secretory fibers 

 proper, and therefore give some flow of secretion on stimulation. 

 These variations might also be expected to be more marked when 



