

39 8 TEXT-BOOK OF PHYSIOLOGY. 



ules which apparently are the antecedents of the digestive enzymes, 

 granules of glycogen, globules of fat, sugar, and proteid, as in the 

 case of the mammary gland. In how far all these compounds are 

 the result of secretory activity or of a cell degeneration and disinte- 

 gration it is impossible to state in the light of present knowledge. 

 During the period of gland rest the blood-supply to the gland is 

 merely sufficient for nutritive purposes. When the occasion arises 

 for gland activity, the blood-vessels, under the influence of the 

 vasomotor nerves, dilate and deliver to the gland an amount of blood 

 far beyond that required for nutritive purposes. As a result the gland 

 becomes red and vascular and the blood emerging by the veins fre- 

 quently retains its customary arterial color. The increased blood-supply 

 favors a rapid transudation of water and salts into the lymph-spaces 

 where they are speedily absorbed and transmitted by the epithelial 

 cells into the interior of the gland lumen. Coincident with the passage 

 of water through the cell, the organic constituents are extruded from 

 the end of the cell bordering the lumen to become dissolved, or in the 

 case of fat to be suspended, in the water. The secretion thus formed 

 accumulates, and with the rise of pressure which inevitably follows 

 it at once passes into the ducts to be discharged on the surface of the 

 mucous membrane or skin, as the case may be. 



Influence of the Nerve System. The activity of every gland 

 is controlled by^ nerve-centers situated in the central nerve system. 

 These centers mayr3e excited to activity either by impressions made 

 on the peripheral terminations or by emotional states, or, possibly, by 

 changes in the composition of the blood itself. As a jrule.allnormal 

 secretion is a. _reflex_act involving the usual mechanisnTT^izrTT^elfitrent 

 surface (skm, mucous membrane, or sense-organ), an afferent nerve, an 

 emissive cell from which emerges an efferent nerve to be distributed 

 to a responsive organ, the gland epithelium. 



For the production of the secretion by the epithelial cell it 

 is believed by some experimenters that two physiologically dis- 

 tinct, efferent nerve-fibers are involved one stimulating the pro- 

 duction of the organic constituents (trophic nerves), the other stimu- 

 lating the secretion of water and inorganic salts (secretory nerves). 

 The evidence for the influence of the nerve system on secretion and 

 the mode of connection of the nerve-fibers with the gland-cells have 

 been alluded to and will again be in subsequent chapters. 



The reticular and vascular glands, though not possessing any 

 common histologic features, are grouped together merely for con- 

 venience, and will be considered in a separate chapter in connection 

 with the problems of internal secretion. 



