348 PRINCIPLES OF GENERAL PHYSIOLOGY 



with secretin. Some of these figures are reproduced in monochrome in Fig. 92 

 (see description of figure). 



If the juice secreted under natural conditions contained active trypsin, it is 

 difficult to understand the use of the production of enterokinase in the intestine. 

 No doubt, however, the flow of water through the cells might carry away zymogen 

 material before it had been worked up by the cell and this would require 

 activation. 



It is not only glands with visible secretion that are under the control of the 

 nervous system, but also those of internal secretion. The fact has been shown 

 especially in the case of the adrenals. When the splanchnic nerves are excited in 

 any way, there is an output of adrenaline into the blood, which produces the 

 various phenomena due to stimulation of the sympathetic, such as rise of blood 

 pressure, etc. (see Asher, 1910; Elliott, 1912, etc.). 



THE SECRETORY PROCESS 



It seems evident that there are two kinds of processes, or rather two factors, at 

 work ; one concerned with the transfer of water, together with certain solutes 

 already present in blood, the other concerned with the elaboration of new chemical 

 compounds. Whether either of these can be excited without the other, by means 

 of specific nerve fibres or by chemical means, it is at present impossible to state. 

 It is to be remembered that the passage of water in itself would wash out 

 constituents of gland cells previously stored therein, but the results of vagus 

 stimulation on the pancreas indicate that new chemical changes can also be set in 

 action by nerve influence. In the idea of " trophic " nerves, Heidenhain appears 

 to include the function of exciting the formation or replacement of the substances 

 which had been given off from the cells in the process of secretion previously. 

 The vagus effect, described above, suggests rather the setting into play of a 

 chemical change in the products already stored in the cells. Secretin, on the 

 other hand, apparently sets going a process by which water washes out stored 

 substances without change. The prolongation of the period of increased 

 oxygen consumption considerably beyond the actual period of secretion itself, 

 induced by the stimulation of nerves, suggests that the restitution process, by 

 which the cells are restored to a state ready for renewed activity, is an automatic 

 process and controlled by mass action in a reversible system. In the moderated 

 natural process of secretion, such as that of the pancreas induced by the intro- 

 duction of acid into the duodenum, the fact that signs of fatigue appear in the 

 cells only after very prolonged activity shows that the natural process of 

 restitution keeps pace with the secretory activity of the cells. 



On the whole, it appears that the usual process of secretion is somewhat as 

 follows : During the period of rest, the cells build up compounds which are 

 preliminary stages of constituents of the secretion, which is afterwards set going 

 by excitation, nervous or chemical. The formation of this material is probably 

 a reversible reaction, so that, after a time, further production ceases, owing to 

 accumulation of products. When the gland is excited to activity, a current of 

 water is set flowing through the cell by some means, probably of an osmotic nature 

 and effected by a combination of increased permeability of the outer end of the 

 cell together with splitting up of some substance into smaller molecules. This 

 current of water washes out into the duct the substances of the secretion already 

 stored in the cell, sometimes after they have been further changed by a process 

 which does not take place until the cells are excited to secretory activity. As 

 the stored substances are lost from the cell, there will be a renewed formation to 

 re-establish equilibrium ; so that, if the activity is not too violent, there will be 

 a balance between the amount secreted and its new formation. Continuous 

 secretion will thus be possible without fatigue. It will be seen that, on this view, 

 the increased production in the cell of the substances which give rise afterwards to 

 the actual products contained in the secretion is not to be supposed to be under 

 the control of the nervous system or other excitatory influence, but that it is a 

 spontaneous activity of the cell itself, controlled by chemical equilibrium. Thus 



