PROCESS OF SECRETION. 321 



prostate. These various organs differ from each other only 

 in secondary points of structure ; such as, chiefly, the arrange- 

 ment of their excretory ducts, the grouping of the acini and 

 lobules, their connection by fibro-cellular tissue, and supply of 

 bloodvessels. The acini commonly appear to be formed by a 

 kind of fusion of the walls of several vesicles, which thus com- 

 bine to form one cavity lined or filled with secreting cells 

 which also occupy recesses from the main cavity. The small- 

 est branches of the gland-ducts sometimes open into the cen- 

 tres 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, 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. 105), such as the 

 kidney and testis, form another division. These consist of tu- 

 bules 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 

 glands are alike in some essential points, besides those which 

 they have in common with all truly secreting structures. They 

 agree in presenting a large extent of secreting surface within 

 a comparatively small space ; in the circumstance that while 

 one end of the gland-duct opens on a free surface, the oppo- 

 site end is always closed, having no direct communication with 

 bloodvessels, or any other canal ; and in uniform arrange- 

 ment of capillary bloodvessels, ramifying and forming a net- 

 work around the walls and in the interstices of the ducts and 

 acini. 



PROCESS OF SECRETION. 



From what has been said, it will have already appeared 

 that the modes in which secretions are produced are at least 

 two. Some fluids, such as the secretions of serous membranes, 

 appear to be simply exudations or oozings from the bloodves- 

 sels, whose qualities are determined by those of the liquor san- 

 guinis, while the quantities are liable to variation, or are 

 chiefly dependent on the pressure of the blood on the interior 



