404 SECRETIOX. 



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 struc- 

 tures. They agree in presenting a large extent of secreting 

 surface within a comparatively small space ; in the circum- 

 stance that while one end of the gland-duct opens on a 

 free surface, the opposite end is always closed, having 

 no direct communication with blood-vessels, or any other 

 canal ; and in an uniform arrangement of capillary blood- 

 vessels, ramifying and forming a network 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 mem- 

 branes, appear to be simply exudations or oozings from the 

 blood-vessels, whose qualities are determined by those of 

 the liquor sanguinis, while the quantities are liable to 

 variation, or are chiefly dependent on the pressure of the 

 blood on the interior of the blood-vessels. But, in the 

 production of the other secretions, such as those of mucous 

 membranes and all glands, other besides these mechanical 

 forces are in operation. Most of the secretions are indeed 

 liable to be modified by the circumstances which affect 

 the simple exudation from the blood-vessels, and the pro- 

 ducts of such exudations, when excessive, are apt to be 

 mixed with the more proper products of all the secreting 

 organs. But the act of secretion in all glands is the result 

 of the vital processes of cells or nuclei, which, as they 

 develop themselves and grow, form in their interior the 



