THE LIVER SECRETION OF BILE. 511 



which occurs in other organs is, that a portion of the product is, from the 

 anatomical condition of the part, thrown out of the system." The Secreting 

 Cells of different organs have the power of elaborating a great variety of pro- 

 ducts ; and' no more essential differences can be discovered in their character, 

 than in the structure of the glands into whose composition they enter. We 

 are entirely ignorant, therefore, of the reason why one set of cells should 

 secrete Bile, another Urea, another a colouring substance, and so on ; but we 

 are as ignorant of the reason why, in the parti-coloured petal of a Flower, the 

 cells of one portion should secrete a red substance, whilst those in immediate 

 contact with it form a yellow or blue colouring matter ; and we know as little 

 of the cause which occasions one set of the Cells of the Embryo to be con- 

 verted into Muscular tissue, another into Cartilage, and so on. Although, 

 therefore, there is a limit to our knowledge, beyond which it does not at pre- 

 sent seem probable that we shall ever pass, it is an important step in the 

 Science of Physiology to have attained the general principle, that in Animals, 

 as in Plants, the act of Secretion is effected by the process of Cell-growth. 



652. It is important to' bear in mind that an essential difference exists 

 between the vital power concerned in the true Secreting process, and the 

 physical property which occasions fluid Exhalation or Transudation. This 

 difference 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 

 depends merely upon the physical condition of the parts, in regard to the 

 amount and the nature of the fluid it contains, and the permeability of its tis- 

 sues. 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 ( 463), 

 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 

 diminished resistance in their walls. -We shall hereafter see, in examining 

 the Physiology of the Urinary secretion, a very striking example of the con- 

 trast between physical Transudation and vital Secretion ( 668). 



II. The Liver. Secretion of Sile. 



653. The Liver is probably more universally found, throughout the Animal 

 scale, than any other gland. Its form varies so greatly, however, in different 

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

 posed to question whether any identity of character exists amongst the several 

 organs, which we include under this designation. In the higher Polypes, for 

 example, we find it to consist of a number of distinct follicles, 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 secre- 

 tion than by any other distinction, that these follicles are recognized as Hepa- 

 tic. In the lower Articulata, a very similar confirmation 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 Respiration of these beings, which renders unnecessary any 

 other complex apparatus for elaborating carbon from the system. -On the other 



