GENERAL CONSIDERATIONS. 529 



We come, then, to a comparison of the relative effects of the cranial and 

 sympathetic nerves as the final part of the evidence for the existence of 

 two classes of nerve-fibres. It is said that the difference in the percentage 

 composition of sympathetic saliva, and of that produced by stimulating 

 the cranial nerve, can only be satisfactorily explained by supposing that 

 secretory and trophic fibres are present in both, and that the number 

 of trophic fibres relatively to the secretory is greater in the sympathetic 

 than in the cranial nerves. 



This conclusion seems to me to be legitimate and unavoidable, if a 

 diminution in the blood supply to the glands brought about by vaso- 

 constrictor nerves does not markedly increase the percentage of organic 

 substance in the saliva secreted. But this, so far, has not been shown 

 to be the case (cf. p. 508). The question can hardly be settled until 

 means are found of stimulating the sympathetic vaso-constrictor fibres 

 of the salivary glands without stimulating the sympathetic secretory 

 fibres. 



We find, then, that the hypothesis of a separate class of trophic 

 fibres, although affording a convenient explanation of a certain number 

 of facts, can hardly be considered proved at any point. It presents also 

 certain difficulties of its own which we need not insist on here. 



On the whole, I think the most probable view is, that only one 

 kind of nerve-fibre runs to the gland-cells, and that this causes all the 

 changes in the gland-cells which are capable of being caused by nerve 

 stimulation. These changes include the taking up proteid material 

 from the lymph, some katabolic action shown by the setting free of 

 carbonic acid and changes leading to the passage of water and salts 

 through the cell. It is not improbable that the nervous impulses hasten 

 the conversion of absorbed proteid to secretory substances, and it is 

 perhaps possible that they increase the solubility of the secretory sub- 

 stance already formed. The effect of the secretory fibres, as regards 

 the amount and percentage composition of the saliva obtained, would 

 naturally vary with the strength of the stimulus, the condition of the 

 gland at the time, the quality and quantity of the blood flowing through 

 the gland. 



The exact processes which take place in gland-cells and which lead 

 to secretion is at present outside the range of our knowledge. The high 

 secretory pressure naturally suggests osmosis as the cause of the passage 

 of water and of salts. And, about five and twenty years ago, the view 

 that secretion is due to the formation in the cells of a substance of high 

 endosmotic pressure was put forward by Hering and others. Much 

 more is known now of the phenomena of osmosis than was known then ; 

 but the nature of the process is still so obscure, that to attempt to 

 explain secretion on the lines of osmosis is to venture on little better 

 than conjecture. 



It may, however, be worth while to state briefly some points regarding the 

 relation, or possible relation, of osmosis to secretion. 



We will consider, first, what facts of secretion we could in some sort 

 account for, on the theory that osmotic pressure is of the same nature as 

 gaseous pressure, and assuming that osmosis does take place in the gland- cells. 



The facts which it seems most feasible to offer an explanation of are, the 

 occurrence of secretion when the cells are stimulated and not at other times, 

 the increase in the rate of flow during stimulation, the increase in the per- 

 centage of salts in saliva with increase in the rate of flow. 

 VOL. i. 34 



