302 HANDBOOK OF PHYSIOLOGY. 



more or less viscid, grayish, or semi-transparent fluid, of alkaline reac- 

 tion and high specific gravity, named mucus. It mixes imperfectly 

 with water, but, rapidly absorbing liquid, it swells considerably when 

 water is added. Under the microscope it is found to contain epithelium 

 and leucocytes. It is found to be made up, chemically, of mucin, 

 which forms its chief bulk, of a little albumen, of salts chiefly chlorides 

 and phosphates, and water with traces of fats and extractives. 



SECRETING GLANDS. 



The secreting glands present, amid manifold diversities of form and 

 composition, a general plan of structure; all contain, and appear con- 

 structed with particular regard to the arrangement of the cells, which, as 

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

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



Types of Accreting Glands. Secreting glands may be classified accord- 

 ing to certain types, which are the following : 1. The simple tubular 

 gland (A, fig. 220), examples of which are furnished by the follicles of 

 Lieberkiihn, and the tubular glands of the stomach. They are 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 tor- 

 tuous sudoriferous glands. 



2. The compound tubular glands (L>, fig. 220) form another division. 

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

 gland may be made up 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, testes, salivary glands, pan- 

 creas, 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 aggregates racemose glands, in which a number of vesicles or 

 acini are arranged in groups or globules (c, fig. 220). The meibomian 

 follicles are examples of this kind of gland. There seem to be glands of 

 mixed character, combining some of the characters of the tubular with 

 others of the racemose type; these are called tubulo-racemose or tubulo- 

 acinous glands. These glands 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 



