38 NATURE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



that which occurs after the death of the entire structure. But in Ani- 

 mals it is much more complicated ; owing to the larger number of con- 

 stituents in their fabric, and to the much greater variety in the propor- 

 tions in which these are combined ; hence the products of interstitial 

 decomposition are much more numerous and varied, and several distinct 

 modes are devised for getting rid of them. Moreover, as the animal 

 tissues are much further removed than the Vegetable from the composi- 

 tion of Inorganic bodies, they are subject to much more rapid and con- 

 stant decay ; and we shall find that this decay is so considerable in 

 amount, as to require on the one hand a very complex excretory appa- 

 ratus to carry off the disintegrated matter, and on the other a large 

 supply of nutrient materials to replace it. 



41. The preceding history may be thus summed up. I. The Vege- 

 table cell-germ or reproductive molecule draws to itself, and combines 

 together, certain inorganic elements ; and thereby produces a new and 

 peculiar compound. This compound, however, exhibits no properties that 

 distinguish it from others, in which ordinary Chemical agencies have 

 been concerned ; and we may, therefore, regard the first act of the cell- 

 germ as of a purely chemical nature. We shall presently see that che- 

 mical agencies are undoubtedly concerned in it, to a very considerable 

 degree. The Animal cell-germ does not possess the same Chemical 

 power ; it is not capable of decomposing the water, carbonic acid, and 

 ammonia, which include the elements of its tissues ; and it is entirely 

 dependent for its growth, upon the supply of nutriment previously pre- 

 pared for it by the agency of the vegetable kingdom, many species of 

 which possess the power of generating a large amount of proteine-com- 

 pounds within their cells, though they do not organize them. II. The 

 cell-germ then exerts an Assimilating agency upon the pabulum thus 

 prepared ; by which a new arrangement of its particles is produced. 

 This new arrangement gives new and peculiar qualities to the fluid, 

 which show that it is something more than a mere chemical compound, 

 and that it is in the act of undergoing the process of organization, ill. 

 The Formation of this elaborated pabulum into tissue then takes place ; 

 its materials are withdrawn from the fluid, and incorporated with the 

 solid texture ; and in thus becoming part of the organized fabric, they 

 are caused to exhibit its own peculiar properties. IV. At the same 

 time, another portion of this pabulum is gradually prepared to serve as 

 the germ of a new cell, or set of cells, by which the same properties are 

 to be exhibited in another generation, v. By an operation resembling 

 that concerned in the first preparation of the pabulum, certain products, 

 more or less differing from it in character, but not destined to undergo 

 organization, are formed in the cavity of the cell. VI. A decomposition 

 or disintegration of the organized structure is continually going on, by 

 the separation of its elements into simpler forms, under the influence of 

 purely Chemical agencies ; and the setting free of these products by an 

 act of excretion, is thus incessantly restoring to the Inorganic world a 

 portion of the elements that have been withdrawn from it. VII. When 

 the term of life of the parent-cell has expired, and its reproductive 

 molecules are prepared to continue the race, the actions of nutrition 

 cease ; those of decomposition go on unchecked ; and the death of the 



