CHAPTER XXIX 

 THE DIGESTIVE SECRETIONS AND THEIR CONTROL 



Organs of Secretion. The simplest form in which a secreting 

 organ occurs (A, Fig. 137) is that of a flat membrane provided with 

 a layer of cells, a, on one side (that on which the secretion is poured 

 out) and with a nelbwork of capillary blood-vessels, c, on the other. 

 The dividing membrane, b, is known as the basement membrane 

 and is usually made up of flat, closely fitting connective-tissue 

 corpuscles; supporting it on its deep side is a layer of connec- 

 tive tissue, d, in which the blood-vessels and lymphatics are sup- 

 ported. Such simple forms of secreting surfaces are found on the 

 serous membranes, but are not common; in most cases an extended 

 area is required to form the necessary amount of secretion, and if 

 this were attained simply by spreading out plane surfaces, these 

 from their number and extent would be hard to pack conveniently 

 in the Body. Accordingly in most cases, the greater area is at- 

 tained by folding the secreting surface in various ways so that a 

 large area can be packed in a small bulk, just as a Chinese lantern 

 when shut up occupies much less space than when extended, al- 

 though its actual surface remains of the -same extent. In a few 

 cases the folding takes the form of protrusions into the cavity of 

 the secreting organ as indicated at C, Fig. 137, and found on some 

 synovial membranes; but much more commonly the surface ex- 

 tension is attained in another way, the basement membrane, cov- 

 ered by its epithelium, being pitted in or involuted as at B. Such 

 a secreting organ is known as a gland. 



Forms of Glands. In some cases the surface involutions are 

 uniform in diameter, or nearly so, throughout (B, Fig. 137). Such 

 glands are known as tubular; examples are found in the lining coat 

 of the stomach (Fig. 129); also in the skin (Fig. 142), where they 

 form the sweat-glands. In other cases the involution swells out at 

 its deeper end and becomes more or less sacculated; (E) such 

 glands are racemose or acinous. The small glands which form the 

 oily matter poured out on the hairs are of this type. In both kinds 



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