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in connection with the production of lymph. They include the physical 

 processes, diffusion, osmosis, and filtration combined with a secretor activity 

 of the cells of the capillary wall. The question as to which of these pro- 

 cesses is the more active is yet a subject of investigation. 



As the chemic composition and the chemic features of the organic con- 

 stituents of all secretions have been demonstrated to be the outcome of 

 metabolic processes going on within the epithelial cells, it must be assumed 

 at least that these differences are correlated with differences in the histo- 

 logic features and chemic composition of the epithelium. The discharge of 

 the secretion is, as a rule, intermittent; that is, there are periods of inactivity 

 or rest. In rest more especially the epithelial cells, after the assimilation 

 of lymph, accumulate within themselves such characteristic products as 

 globules of mucin, granules which apparently are the antecedents of the diges- 

 tive enzymes, granules of glycogen, globules of fat, sugar, and proteins, 

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

 the result of secretor activity or of a cell degeneration and disintegration 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 vaso-motor 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 frequently retains its customary arterial color. The increased blood- 

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

 from which 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. 



The Influence of the Nerve System. The activity of every gland is con- 

 trolled by nerve-centers situated in the central nerve system. These centers 

 may be excited to activity either by impressions made on the peripheral 

 terminations of afferent nerves or by emotional states, or, possibly, by changes 

 in the composition of the blood itself. As a rule, all normal secretion is a 

 reflex act involving the usual mechanism: viz., a receptive surface (skin, 

 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, though the secretion may in some instances be initiated 

 by a psychic state. 



For the production of the secretion by the epithelial cell it is believed 

 by some experimenters that two physiologically distinct, efferent nerve- 

 fibers are involved one stimulating the production of the organic constituents 

 (trophic nerves), the other stimulating the secretion of water and inorganic 

 salts (secretor 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 (page 156) and will again be in subsequent chapters. 



The structure of the glands of external secretion, the composition and 



