SECRETION. 397 



a mucin-like substance, a nucleo-albumin, which imparts to it con- 

 siderable viscidity. This synovial fluid serves to diminish friction 

 between the opposing surfaces of the bones as they glide over one 

 another during movement. 



Other secretions, such as the aqueous and vitreous humors of the 

 eye, the fluid of the internal ear, the cerebrospinal fluid, etc., will be 

 considered in connection with the organs with which they are asso- 

 ciated, as have been the digestive secretions. 



The secreting glands are formed of the same histologic elements 

 as the secreting membranes. They are formed by an involution of 

 the mucous 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 lymphatics 

 and nerves, of which the latter are in direct connection with the blood- 

 vessels and epithelial cells. The interior of each gland is in com- 

 munication with the free surface by one or more passageways known 

 as ducts. 



/tliese 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 A fundus glands of the stomach; or compound 

 e.~g., kidney, jesticle, salivary, and lachrymal glands. 



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

 the sebaceous glands, the ovarian follicles, meibomian glands; or 

 compound, as the mammary glancTs^arid salivary glands. 



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

 of the blood, the common material, be delivered to the lymph-spaces f 

 with which the epithelial cells are in close relation. The processes! 

 involved in the passage of the plasma across the capillary wall have 

 already been considered in connection with the production of lymph. 

 They include the physical processes, diffusion, filtration, and osmosis, 

 combined with a secretory activity of the cells of the capillary 'wall. 

 The question as to which of these processes is the more active is yet 

 a subject of investigation. 



As the chemic composition and the chemic features of the organic 

 constituents of all secretions have been demonstrated to be the out- 

 come of metabolic processes going on within the epithelial cells, it 

 must be assumed at least that these differences are correlated with 

 differences in the histologic features and molecular structure of the 

 epithelium. The discharge of the secretion is, as a rule, intermittent ; 

 that is, there are periods of activity on the part of the gland fol- 

 lowed by 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, gran- 



