THE SECRETORY TISSUES AND ORGANS. 297 



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 compared 

 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 contraction of the blood-vessels. 

 If this were so, cutting off or diminishing the blood-supply 

 of the gland in any way ought to have the same result as 

 stimulation 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 cere- 

 bral gland-nerve stimulated, it is found that the percentage of 

 organic constituents in the secretion is as low as usual; it re- 

 mains 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-constrictor 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 others. Primarily 

 dependent upon filtration and dialysis from the blood-vessels 

 and upon the physiological character of the gland-cells, both 

 of these factors are, we find, controlled by the nervous system, 

 such secreting cells being no more automatic than striped 

 muscle; and the facts also give us important evidence of the 

 power of the nervous system to influence cell nutrition directly. 

 In other simpler cases, secretion seems to be a mere direct re- 

 sult of the growth and life of the secreting cell; for example 

 the formation, storage and discharge of fatty matters by the 

 oil-glands of the skin. 



Summary. By secretion proper 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 fil- 

 tration and dialysis; for example, the elimination of carbon 

 dioxide from the surfaces of the lungs, and very watery liquid 

 poured out on the surface of the serous membranes. Such 

 secretions are known as transudata, and their amount is only 



