574 ORGANS OF SECRETION. 



surface of internal organs. The secerning membranes most 

 generally present a tubular form, open at one end, and closed 

 at the other, forming cells, or sacs, or follicles, or tubuli, or 

 conglobate, or conglomerate glands, by which the largest 

 secreting surface is accommodated in the smallest space, and 

 the readiest transmission is afforded to the secreted matter ; 

 and the whole form and structure of these glands appear to 

 become more complex, as the fluids they produce differ more 

 from the constitution of the blood which affords them. 



The alimentary canal traversing the interior of animals, 

 itself lined with a secreting membrane, is the common pas- 

 sage for foreign matter through their body, and into which 

 the most numerous streams of secreted fluids are incessantly 

 pouring from all sides. Most of the glandular secretions 

 poured into this canal, assist in the conversion of this ma- 

 terial into chyle, to replenish the blood ; and these chy- 

 lopoietic glands, the most essential to life and growth, are 

 the first formed in the animal body, and the most universal 

 in their occurrence in the animal kingdom. Indeed, the 

 whole processes of the nutrition and growth of every part 

 of the organization, though not effected by distinct glands, 

 may be considered as a series of secretions of their own 

 materials, by all the different constituent tissues of the be dy. 

 The first-formed secreting surface, and the most universal in 

 animals, is the skin, and next, its internal continuation, the 

 digestive canai ; and nearly every distinct gland of the body, 

 is developed from one of these two surfaces, which in most 

 of the lower animals, are very similar to each other. 



A mucous membrane, a mucous follicle, and a muciparous 

 gland, are but different forms of the same surface, more or 

 less extended, and more or less isolated, and adapted alike, 

 under every form, for the distribution of capillary blood- 

 vessels, and for affording their peculiar secretion. When 

 these common secreting membranes have acquired a com- 

 plicated and isolated form, so as to present, in small space, 

 a large surface for the distribution of the capillaries, which 

 afford the materials of the secretion, they constitute glands, 

 which may thus be developed as isolated follicles, or the 

 most ramified tubuli, from every mucous^ cutaneous, or serous 

 surface of the body. The adult forms of all glands in the 

 lowest tribes of animals, and the earliest transient rudi- 



