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GENERA L ANA TOM Y. 



SECRETING GLANDS. 



The secreting glands are organs whose cells manufacture a secretion of a more 

 or less definite composition, the material for the secretion being primarily selected 

 from the blood. The essential parts, therefore, of a secreting gland are 

 cells, which have the power of extracting from the blood certain matters, 

 and in some cases converting them into new chemical compounds ; and blood- 

 vessels, by which the blood is brought into close relationship with these cells. 

 The general arrangement in all secreting structures that is to say, not only 

 in secreting glands, but also in secreting membranes is that the cells are 

 arranged on one surface of an extravascular basement-membrane, which supports 

 them, and a minute plexus of capillary vessels ramifies on the other surface of the 

 membrane. The cells then extract from the blood certain constituents which pass 

 through the membrane into the cells, where they are prepared and elaborated. 

 The basement-membrane does not, however, always exist, and any free surface 

 would appear to answer the same purpose in some cases. 



By the various modifications of this secreting surface the different glands are 

 formed. This is generally effected by an invagination of the membrane in different 

 ways, the object being to increase the extent of secreting surface within a given 

 bulk. 



In the simplest form a single invagination takes place, constituting a simple 

 gland ; this may be either in the form of an open tube (Fig. 71, A), or the walls 



FIG. 71. Diagrammatic plan of varieties of secreting glands. A. Simple gland. B. Sacculated simple gland, 

 c. Simple convoluted tubular gland. D, E. Racemose gland. F. Compound tubular gland. 



of the tube may be dilated so as to form a saccule (Fig. 71, B). These are named 

 the simple tubular or saccular glands. Or, instead of a short tube, the invagination 

 may be lengthened to a considerable extent, and then coiled up to occupy less 

 space. This constitutes the simple convoluted tubular gland, an example of which 

 may be seen in the sweat-glands of the skin (Fig. 71, c). 



If, instead of a single invagination, secondary invaginations take place from 



