618 OF SECRETION. 



single follicle ; their size increasing, and the character of their contents becoming more dis- 

 tinct, in proportion to their distance from the germinal spot (which is at the blind termina- 

 tion of the follicle), and their consequent proximity to the outlet (Fig. 41). In some varieties 

 of such glands, however, especially when the follicles are extended into prolonged tubes, the 

 production of new cells does not take place from a single germinal spot at the extremity of 

 the follicle, but from a number of points scattered through its entire length. 



b. In the second type of Glandular structures, the parent-cell does not remain as a perma- 

 nent follicle; but, having come to maturity, and formed a connection with the excretory 

 duct, it discharges its entire contents into the latter, and then shrivels up and disappears, to 

 be replaced by newly-developed follicles. In each parent-cell of a gland formed upon this 

 type, we shall find all its secondary or secreting cells at nearly the same grade of develop- 

 ment ; but the several parent-cells, of which the parenchyma of the gland is composed, are 

 in very different stages of growth at any one period some having discharged their contents, 

 and being in progress of disappearance, whilst others are just arriving at maturity, and con- 

 necting themselves with the excretory duct ; others exhibiting an earlier degree of develop- 

 ment in the secondary cells ; others presenting the latter in their incipient condition ; whilst 

 others are themselves just starting into existence, and as yet exhibit no traces of the second 

 generation, which they are destined subsequently to develope. 



c. The former of these seems to be the usual type of the ordinary glands ; the latter is 

 chiefly, if not entirely, to be met with among the Spermatic Glands.* 



824. It is important to bear in mind, that an essential difference exists be- 

 tween the vital power concerned in the true Secreting process, and the phy- 

 sical property which occasions fluid Exhalation or Transudation. This dif- 

 ference is precisely the same as that which exists between the vital act of 

 Selective Absorption, and the physical operation of Endosmose or Imbibition. 

 By Imbibition and Transudation, certain fluids may pass through organic 

 membranes, in the dead as well as in the living body ; and this passage de- 

 pends merely upon the physical condition of the part, in regard to the amount 

 and the nature of the fluid it contains, and the permeability of its tissues. 

 Not only does water thus transude, but various substances that are held in 

 complete solution in it, especially albumen and saline matter: it is in this 

 manner that the Blood absorbs fluids from the digestive cavity ( 675), and 

 pours out the serous fluid which occupies the interspaces of the areolar tissue 

 and the serous cavities. The transudation of the watery portion of the blood 

 is much increased by any impediment to its flow through the vessels, as in 

 Congestion and Inflammation; and also by any causes that produce a dimi- 

 nished resistance in their walls. We shall hereafter see, in examining the 

 Physiology of the Urinary secretion, a very striking example of the contrast 

 between physical Transudation and vital Secretion ( 840). 



2. The Liver. Secretion of Bile. 



825. The Liver is probably more universally found, throughout the Ani- 

 mal scale, than any other gland. Its form varies so greatly, however, in dif- 

 ferent tribes, that, without a knowledge of its essential structure, we should be 

 disposed to question whether any identity of character exists amongst the 

 several organs which we include under this designation. 



a. In the higher polypes, for example, we find it to consist of a number of distinct folli- 

 cles, lodged within the walls of the stomach, and pouring their secretion into its cavity by 

 as many separate orifices; and it is more by the peculiar character of their secretion, than 

 by any other distinction, that these follicles are recognized us Hepatic. In the lower Articu- 

 lata, a very similar conformation is met with ; but in the higher classes of this series, such 

 as Insects, the follicles are prolonged into tubes of considerable extent. It is very curious 

 to observe, in animals of such complex structure, that a few long tubes, closed at one end, 

 and opening at the other into the alimentary canal, are all which they have to represent a 

 Liver ; but the wonder is readily accounted for by keeping in view the extremely active 



See Goodsir's Anatomical and Pathological Researches, Chap. v. 



