GLANDS. 389 



cells representing this epithelium have been already described 

 in a former chapter of this work. 



Following up this general definition of glandular structure, 

 a gland may be defined as a collection or aggregation of gra- 

 nular cells placed in close approximation with a formative 

 fluid, contained, ordinarily, but not essentially, in the blood 

 vessels. 



This definition of a gland embraces not merely those 

 glands which are provided with orifices or excretory ducts, 

 but also those which have no such external or apparent chan- 

 nels of excretion, as the vascular and other glands. 



While it is certain that secretion takes place in the cavities 

 of the granular cells, as may be actually shown to occur in 

 the hepatic, renal and sebaceous cells, there is much reason to 

 believe from the rapid and continual development and dissolu- 

 tion of successive generations of cells as especially evident in 

 the case of those of the stomach and small intestines, that this 

 secretion is liberated in many instances only by the dissolu- 

 tion of cells themselves. From this view of the relation 

 existing between the secretion and the cells, it would ap- 

 pear that the secretive process is accompanied by an en- 

 dosmotic action. 



In the case of fat cells, the secretion is rarely eliminated, 

 the secretive process is exceedingly slow, and the oleaginous 

 matter is stored up and retained within the cavities of the 

 cells themselves, the growth of the cell membrane keeping 

 pace with the increase in the amount of its fatty contents. 



The idea of fluidity is almost inseparably connected with 

 our notions of a secretion : secretions, however, are not 

 essentially fluid, for there are solid secretions as well as fluid ; 

 the sebaceous matter of the glands of the prepuce is a solid, 

 and the urine of many serpents, as the boa, is also stated to 

 be solid. There is little doubt, however, but that most and 

 perhaps all secretions are fluid during the act of their forma- 

 tion, and that the solidity acquired by certain of them occurs 

 after their perfection and elimination. 



In a strictly systematic arrangement of structures, the 

 majority of the glands might well be described in connexion 



