INFLUENCE OF NERVES ON SECRETION. 275 



though unable of itself to cause a secretion, brings about 

 great chemical changes in the gland-cells. It is a distinct 

 trophic nerve. This conclusion is confirmed by histology. 

 Sections of the gland after prolonged stimulation of the sym- 

 pathetic show its cells to be quite altered in appearance., 

 and in their tendency to combine with carmine, when com- 

 pared either with those of the resting gland or of the gland 

 which has been made to secrete by stimulating its glosso- 

 pharyngeal branch alone. 



We have still to meet the objection that the sympathetic 

 fibres may be only indirectly trophic, governing the meta- 

 bolism of the cells through the blood-vessels. If this be 

 so, cutting off or diminishing the blood-supply of the 

 gland, in any way, ought to have the same result as stimula- 

 tion of its sympathetic fibres. Experiment shows that 

 such is not the case and reduces us to a direct trophic influ- 

 ence of the nerve. When the arteries are closed and the 

 cerebral gland-nerve stimulated, it is found that the per- 

 centage of organic constituents in the secretion is as low 

 as usual; it remains almost exactly the same whether the 

 arteries are open or closed or have been previously open or 

 closed. We must conclude that the peculiar influence of 

 the sympathetic does not depend upon its vaso-con stricter 

 fibres. 



These observations make it clear that the phenomena of 

 secretion are dependent on very complex conditions, at least 

 in the salivary glands and presumably in all others. 

 Primarily dependent upon filtration and dialysis from the 

 blood-vessels and the physiological character of the gland- 

 cells, both of these factors are controlled by the nervous 

 system, the secretory tissues being no more automatic than 

 the muscular; and the facts also give us important evidence 

 of power of the nervous system to influence cell nutrition 

 directly. 



Summary. By secretion is meant the separation of such 

 substances from the blood as are poured out on free surfaces 

 of the Body, whether external or internal. In its simplest 

 form it is merely a physical process dependent on filtra- 

 tion and dialysis; for example, the elimination of carbon 

 dioxide from the surfaces of the lungs, and the watery 



