408 SECRETION. 



2. The aggregated glands, including those that used to be 

 called conglomerate, in which a number of vesicles or acini are 

 arranged in groups or lobules (c, fig. 101). Such are all those 

 commonly called mucous glands, as those of the trachea, 

 vagina, and the minute salivary glands. Such, also, are 

 the lachrymal, the large salivary and mammary glands, 

 Brunn's, Cowper's, and Duverney's glands, the pancreas 

 and prostate. 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 fibre-cellular 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 ; some- 

 times the acini are clustered round the extremities, or by 

 the sides of the ducts : but, whatever secondary arrange- 

 ment there may be, all have the same essential character 

 of rounded groups of vesicles containing gland-cells, and 

 opening, either occasionally or permanently, by a common 

 central cavity into minute ducts, which ducts in the large 

 glands converge and unite to form larger and larger 

 branches, and at length, by one common trunk, open on a 

 free surface of membrane. 



3. The convoluted tubular glands (D, fig. 101), such as the 

 kidney and testis, form another division. These consist of 

 tubules of membrane, lined with secreting cells arranged 

 like an epithelium. Through nearly the whole of their 

 long course, the tubules present an almost uniform size and 

 structure ; ultimately they terminate either in a cul-de-sac, 

 or by dilating, as in the Malpighian capsules of the kidney, 

 or by forming a simple loop and returning, as in the 

 testicle. 



Among these varieties of structure, all the permanent 



