TISSUES, ORGANS, AND SYSTEMS. 115 



ment, in consequence of which the character of these cells as a coating 

 of the glandular canals is frequently lost, and they appear simply as 

 a part of the secretion (testis, lacteal gland during lactation). All the 

 glands here mentioned, with the exception of the sexual, are developed 

 from the internal and external epithelial structures of the body, con- 

 joined with the vascular membranes which support these epithelia. 

 Some of them originate as involutions of these membranes, and retain 

 the cavities throughout the course of their development (lungs, small 

 intestinal glands), others are at first hollow, but afterwards increase by 

 the addition of solid out-growths (liver) ; others, again, are solid from 

 the very first, continue to grow in this condition, and only secondarily 

 come to possess cavities (cutaneous glands, racemose glands). The 

 nutrition of the glands goes on with great energy, and they belong to 

 the most vascular organs of the body. Except in the uterine glands 

 no regeneration of the glandular substance takes place, but hypertrophy 

 occurs in them, and even the accidental formation of minute glands. 



The true glands of the human body may, according to the form of 

 their ultimate elements, above described, be divided as follows : 



1. Glands ivith closed glandular vesicles, which dehisce periodically. 

 Ovary, follicles of the uterus. 



2. Glands whose parenchyma consists of cells united into a network. 

 Liver. 



3. Racemose glands, in which rounded and elongated glandular 

 vesicles are seated upon the ultimate ends of the excretory ducts. 



a. Simple, with one or few glandular lobules. Mucous glands, seba- 

 ceous glands, Meibomian glands. 



b. Composite, with many glandular lobules. Lachrymal glands, sali- 

 vary glands, pancreas, prostate, Cowper's and Bartholini's glands, lac- 

 teal glands, lungs. 



4. Tubular glands, whose secreting elements have the form of 

 canals. 



a. Simple, consisting of only one or a few csecal tubes. Tubular 

 glands of the stomach and intestine, uterine glands, sudoriparous and 

 ceruminous glands. 



b. Composite, with many branched glandular canals, which may also 

 be united into a network. Testis, kidney. 



The forms of the glands of animals, notwithstanding their variety, 

 may, with few exceptions, be brought under one of the four categories 

 here established. The following are worthy of particular notice : 1. 

 The glandular cells, with peculiar excretory ducts, to be found in some 

 Articulata, which either, singly, form glands, or are united together 

 in numbers by a membrana propria. 2. The occurrence of a structure- 

 less, chitinous membrana intima in many glands of the Articulata. 3. 



