330 HANDBOOK OF PHYSIOLOGY. 



ready described in this and previous chapters, we may proceed to con- 

 sider the manner in which they are arranged to form the varieties of 

 secreting glands. 



The secreting glands are the organs to which the function of secretion 

 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 

 expressed, both line their tubes or cavities as an epithelium, and elabo- 

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

 Glands are provided also with lymphatic vessels and nerves. The distri- 

 bution 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 to the secreting cells also in some glands. 



Varieties. 1. The simple tubule or tubular gland (A, Fig. 227), ex- 

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

 Lieberkuhn, 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, is lined with secreting cells arranged as 

 an epithelium. To the same class may be referred the elongated and 

 tortuous sudoriferous glands. 



2. The compound tubular glands (D, Fig. 227) 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, 

 Brunner's glands with the Lachrymal and Mammary glands, and some 

 Mucous glands are examples of this type, but present more or less 

 marked variations among themselves 



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

 acini are arranged in groups or lobules (c, Fig. 227). 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 and 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, 



