OF SECRETION IN GENERAL. 



575 



the skin or mucous membranes 

 that form either the projecting 

 fringes (Fig. 147), or the folli- 

 cles, or extended tubuli (Fig. 

 148), of which the Glandular or- 

 gans are for the most part com- 

 posed ( 235), and are thus readi- 

 ly thrown off from their free sur- 

 faces. Thus, the act of Secre- 

 tion essentially consists in the 

 successive production and exu- 

 viation of the cells which min- 

 ister to it; these cells giving 

 up, by rupture or deliquescence, 

 the substances which they have 

 eliminated from the blood. 

 Each group of cells is adapted 

 to separate a product of some 

 particular kind, which consti- 

 tutes its special pabulum ; and 

 the rate of its production seems 

 to depend cdeteris paribus upon 

 the amount of that pabulum 

 supplied by the circulating fluid 

 ( 120). The substances at 

 the expense of which the se- 

 creting cells grow, however, 

 may not be precisely those 

 which are subsequently cast 

 forth at their death ; for it is 

 very probable that some of them, 

 at least, undergo a certain de- 

 gree of chemical transforma- 

 tion by the agency of these 

 cells; the characteristic mate- 

 rials of the several secretions 

 not always pre-existing as such 

 in the blood. 



618. A distinction may be 

 drawn, as regards this- point, 

 between those Excretions, the 

 retention of whose materials in 

 the Blood would be positively injurious, and those Secretions which are des- 

 tined for particular purposes within the system, and the cessation of which has 

 no immediate influence on any other function than those for which they are re- 

 spectively destined. The solid matter dissolved in the fluids of the latter class 

 is little else than a portion of the nutritive constituents of the blood ; either so 

 little altered as still to retain its nutritive character, as is the case with the 

 casein of Milk, and with the albuminous constituent of the Serous fluid of areolar 

 tissue and of serous and synovial membranes ; or in a state of incipient retro- 

 grade metamorphosis, as seems to be the case with the peculiar " ferments" of 

 the salivary, gastric, pancreatic, and intestinal secretions. On the other hand, 

 the characteristic ingredients of the Excretions are very different in character 

 from the normal elements of the blood. They are all of them completely unor- 

 ganizable ; and they' possess, for the most part, a simple atomic constitution. 



Plan of extension of secreting membrane, by inversion or 

 recession in form of cavities. A, simple glands ; a, 6, c, as in the 

 last figure; g, follicle; h, follicle dilated into a sacculus; i, follicle 

 lengthened into a tubule, which is coiled up. B, multilocular 

 crypts; fc, of tubular form; I, saccular. c, Racemose or vesicular 

 compound glands ; m, entire gland, showing branched duct and 

 lobular structure ; n, a lobule detached, with o, branch of duct 

 proceeding from it. D, Compound Tubular gland. 



