SECRETION 349 



the trophic nerves of Heidenhain are not trophic in the sense of presiding over 

 processes of growth of material, but control the changes in the cell which lead 

 to the transformation of stored substance into the specific organic constituents 

 of the secreted fluid. 



ANABOLIC OR INHIBITORY NERVES 



Under certain conditions, stimulation of the vagus nerve stops a pancreatic 

 secretion in progress, owing to a previous effective excitation, or from injection of 

 secretin. Yon Anrep (unpublished as yet) has investigated this effect and finds 

 that the explanation lies in a contraction of the ducts. It is not surprising that 

 this should be the case, since, as we shall see in the next chapter, the vagus nerve 

 causes contraction of the intestinal muscle, and the pancreatic ducts are outgrowths 

 from the intestine in development. Anrep placed a portion of the pancreas in a 

 plethysmograph and found that, during the cessation of the outward flow of 

 secretion, the gland increased in volume. This latter fact shows that the secretion 

 continued to be formed, but was unable to escape. After a time, the pent-up 

 juice forces its way out and, as the first drop appears, there is a diminution in the 

 volume of the gland, which returns to its normal volume after the apparent 

 inhibition has ceased. It is of interest, also, to note that there is no evidence in 

 these experiments of the presence of vaso-dilator fibres in the vagus, nor of more 

 than a minimal vascular dilatation in the gland when secretin was used, provided 

 that the preparation was free from depressor substance (probably /3-iminazolyl- 

 ethylamine, see Chapter XXIV.). 



Bradford (1888, p. 315) considers that the most satisfactory explanation of the 

 curious phenomenon of the "paralytic secretion" of the submaxillary gland is to 

 be found in the hypothesis of a special set of fibres in the chorda tympani nerve. 

 Their function is to check or inhibit the spontaneous activity of the gland cells. 

 After section of this nerve, u secretion of saliva commences in about four hours 

 and lasts for some time, the gland undergoing atrophy at the same time. Further 

 discussion of the action of inhibitory nerves will be found in Chapter XIII. 



ARTIFICIAL PERFUSION OF GLANDS 



How far the chemical mechanism applies to all glands and whether there are 

 any glands devoid of nervous control, it is not as yet possible to state definitely. 



Although the latter mode of excitation appears to be complete and adequate in 

 the case of the salivary glands, some observations by Demoor (1911, 1912, 1913) 

 show that, in the absence of certain chemical substances, stimulation of nerves is 

 without effect. If the submaxillary gland is perfused with Ringer's solution, 

 oxygenated, excitation of the chorda tympani nerve still brings about vaso-dilatation, 

 but no secretion of saliva. Under the same conditions, the pancreas produces no 

 juice when secretin is added to the perfusion fluid. At first sight, it might be 

 thought that it is impossible to supply sufficient oxygen merely by solution in a 

 saline solution, considering the large consumption of oxygen by the gland cells. 

 That this is not the cause of the complete absence of secretion is shown, however, 

 by the fact that if a certain amount of serum of the same animal (100 c.c. to 

 1,400 c.c. of the saline solution) is added, excitation of the chorda tympani nerve 

 produces a flow of secretion, but only for thirty to sixty seconds. It seems 

 probable that the presence of some constituent of the serum is necessary for the 

 due change in permeability of the cell membrane associated with the process of 

 secretion. The comparatively small amount obtained may arise from the previous 

 store in the cells, and the oxygen supply may be insufficient to afford the energy 

 necessary for the new formation of such substances, or only at a minimal rate. 

 Further observations by Demoor are regarded by him as showing that the way in 

 which a nerve acts in exciting secretion is by causing the production of a chemical 

 substance, which itself acts on the cell processes in a way similar to that in which 

 secretin acts on the pancreas. This exciting substance is perhaps of the nature 

 of a hormone and is carried away in the saliva secreted. The evidence consists 



