440 TEXT-BOOK OF PHYSIOLOGY. 



membrane or skin, the epithelium of which is variously modified structurally 

 and functionally in the various situations in which they are formed. Like 

 the membranes themselves, the glands are invested by capillary blood- 

 vessels and supplied with lymph-vessels and nerves, of which the latter 

 are in direct connection with the blood-vessels and epithelial cells. The 

 interior of each gland is in communication with the free surface by one or 

 more passageways known as ducts. 



These glands may be classified according as the involution is cylindrical 

 or dilated as 



1. Tubular. The tubular glands may be simple e.g., sweat-glands, 

 intestinal glands, fundus glands of the stomach; or compound e.g., kidney, 

 testicle, salivary, and lachrymal glands. 



2. Alveolar. The alveolar glands may also be simple e.g., the seba- 

 ceous glands, the ovarian follicles, meibomian glands; or compound, as 

 the mammary glands and salivary glands. 



For the production of a secretion it is necessary that the plasma of the 

 blood, the common material, be delivered to the lymph-spaces with which 

 the epithelial cells are in close relation. The processes involved in the pass- 

 age of the plasma across the capillary wall have already been considered 

 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 molecular structure 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 protein's, 

 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 



