SIMPLEST FOKM OP GLANDULAR STRUCTURE. 395 



distinctive character depends. All the proper Secretions are thus either 

 poured out upon the exterior of the body, or into cavities provided 

 with orifices that lead to it. Thus we shall see that a considerable 

 quantity of solid matter, and a very large quantity of fluid, of which 

 it is desirable that the system should be freed, are carried off from the 

 Cutaneous surface. Another most important secretion, containing a 

 large quantity of solid matter, and serving also to regulate the quan- 

 tity of fluid in the body, namely, the Urinary, is set free by a 

 channel expressly adapted to convey it directly out of the body. The 

 same may be said of the Mammary Secretion, which is separated from 

 the blood, not to preserve its purity, nor to answer any purpose in the 

 economy of the individual, but to afford nutriment to another being. 

 And of the matters secreted by the very numerous glandulae situated 

 in the walls of the Intestinal canal, a great part are obviously poured 

 into it for no other purpose, than that they may be carried out of the 

 body by the readiest channel. 



714. The cells covering the simple membranes that form the free 

 surfaces of the body, whether external or internal, are all entitled to 

 be regarded as secreting cells, since they separate various products 

 from the blood, which are not again to be returned to it. But the 

 secreting action of some of these seems to have for its object the pro-, 

 tection of the surface ; thus the Epidermic cells secrete a horny matter, 

 by which density and firmness are imparted to the cuticle ; whilst by 

 the epithelial cells of the Mucous membrane of the alimentary canal, 

 and of other parts, their protective Mucus seems to be elaborated. 

 But in general we find that special organs, termed (rlands, are set 

 apart for the production of the chief secretions ; and we have now to 

 consider the essential structure of these organs, and the mode of their 

 operation. A true Gland may be said to consist of a closely-packed 

 collection of follicles, all of which open into a common channel, by 

 which the product of the glandular action is collected and delivered. 

 The follicles contain the secreting cells in their cavities ; whilst their 

 exterior is in contact with a network of blood-vessels, from which the 

 cells draw the materials of their growth and development (Fig. 94). 

 In any one of the higher animals, we may trace out a series of pro- 

 gressive stages of complexity, in the various glands included within 

 their fabric ; and in following any one of the glands that attain the 

 highest degree of development (such as the Liver or Kidney), through 

 the ascending scale of the Animal series, we should trace a very similar 

 gradation from the simplest to the most complex form. 



715. That there is nothing in the form or disposition of the compo- 

 nents of the glandular structure, which can have any influence upon 

 the character of the secretion it elaborates, is evident from the fact, 

 that the very same product, e.g., the Bile, or the Urine, is found 

 to issue from nearly every variety of secreting structure, as we trace 

 it through the different groups of the Animal kingdom. The peculiar 



wer by which one organ separates from the Blood the elements of 

 ;he Bile, and another the elements of the Urine, whilst a third merely 

 seems to draw off a certain amount of its albuminous and saline con- 

 stituents, is obviously the attribute of the ultimate secreting cells, 



