SECRETION. 



The secreting glands are the organs to which the function of seen It <m 

 is more especially ascribed; for they appear to be occupied with it alone. 

 They present, amid manifold diversities of form and composition, a gen- 

 eral plan of structure, by which they are distinguished from all other 

 .textures of the body; especially, all contain, and appear constructed with 

 particular regard to, the arrangement of the cells, which, as already ex- 

 pressed, both line their tubes or cavities as an epithelium, and elaborate, 

 as secreting cells, the substances to be discharged from them. Glands 

 are provided also with lymphatic vessels and nerves. The distribution of 

 the former is not peculiar, and need not be here considered. Nerve -fibres 

 are distributed both to the blood-vessels of the gland and to its ducts; 

 and, in some glands, to the secreting cells also (p. 229). 



Varieties. 1. The simple tubule, or tubular gland (A, Fig. 220), 

 examples of which are furnished by some mucous glands, the follicles of 

 Lieberkiihn (Fig. 186), and the tubular glands of the stomach. These 

 appear to be simple tubular depressions of the mucous membrane, the 

 wall of which is formed of primary membrane, and is lined with secreting 

 cells arranged as an epithelium. To the same class may be referred the 

 elongated and tortuous sudoriferous glands. 



The compound tubular glands (D, Fig. 220) form another division. 

 These consist of main gland-tubes, which divide and subdivide. Each 

 gland may consist of the subdivisions of one or more main-tubes. The 

 ultimate subdivisions of the tubes are generally highly convoluted. 

 They are formed of a basement-membrane, lined by epithelium of various 

 forms. The larger tubes may have an outside coating of fibrous, areolar, 

 or muscular tissue. The kidney, testis, salivary glands, pancreas, Brun- 

 ner's glands with the lachrymal and mammary glands, and some mucous 

 glands are examples of this type, but present more or less marked varia- 

 tions among themselves. 



2. The aggregate or racemose glands, in which a number of vesicles or 

 acini are arranged in groups or lobules (c, Fig. 220). The Meibomian 

 follicles are examples of this kind of gland. 



These various organs differ from each other only in secondary points 

 of structure; such as, chiefly, the arrangement of their excretory ducts, 

 the grouping of the acini arid lobules, their connection by areolar tissue, 

 and supply of blood-vessels. The acini commonly appear to be formed 

 by a kind of fusion of the walls of several vesicles, which thus combine 

 to form one cavity lined or filled with secreting cells which also occupy 

 recesses from the main cavity. The smallest branches of the gland-ducts 

 sometimes open into the centres of these cavities; sometimes the acini 

 are clustered round the extremities, or by the sides of the ducts: but, 

 whatever secondary arrangement there may be., all have the same essen- 

 tial character of rounded groups of vesicles containing gland-cells, and 

 opening by a common central cavity into minute ducts, which ducts in 



