140 OF THE STRUCTURAL ELEMENTS OF THE HUMAN BODY. 



and it lines the bloodvessels and lymphatics, forming the sole constituent of 

 the walls of their minutest divisions. In all these cases, its function appears to 

 be, in part at least, the limitation of the too free transudation of fluid ; a sufficient 

 amount being allowed to pass, however, for the wants of the tissues which it cuts 

 off from direct relation with the blood. Thus, the epidermic and epithelial cells 

 of the skin, mucous membranes, &c., draw their nourishment, through the base- 

 ment-membrane on which they rest, from the vessels of the subjacent tissue; 

 just as the tissues which are themselves permeated by capillary vessels can only 

 draw nutrient materials from the interior of the latter through the membrane 

 which constitutes their walls. But it seems probable that this membrane usually 

 performs a much more important office than that of simply limiting the fluids, 

 whilst allowing the requisite degree of transudation ; for there is strong reason 

 to believe that it has an important instrumentality in the production of the cells 

 which are continually originating on its surface, either by itself furnishing their 

 germs, or (in the case of the epidermis) by promoting the development of cells 

 de novo in the organizable blastema which lies beneath its deepest layers. The 

 mode in which these cells originate, however, will become the subject of dis- 

 cussion hereafter (CHAP. v. SECT. 2). 



2. Of Vital force, and the Conditions of its Exercise. 



120. When we pass from the mere observation of the elementary phenomena 

 of Life, described in the previous section, to the search for their causes, we find 

 that, as in other cases, the conditions of all Vital changes are twofold, namely, 

 material and dynamical (INTRODUCTION, p. 35); the former consisting in the 

 due supply of the appropriate pabulum for the development of organic structure ; 

 the latter, in the exercise of a force or power whereby this is organized and vital- 

 ized. The Animal cell cannot, like that of the Plant, generate its pabulum for 

 itself out of the inorganic elements around it ; but is dependent upon that which 

 has been prepared for it; and in Man, as in all the higher forms of Animal ex- 

 istence, this pabulum is furnished to each growing part by the circulating fluid, 

 which has been prepared by previous operations. It is the essential character 

 of this fluid, that it contains the materials ready to be appropriated by the walls 

 and nuclei of cells, which are probably, as we have seen, of uniform composition ; 

 and the material of the simple fibres and membranes, also, is yielded by the 

 essential components of the blood, in a state in which it seems readily to undergo 

 the requisite transformation. The contents of the various groups of cells form- 

 ing part of the organism are, however, extremely varied; and no particular 

 order of these can be generated, unless the nutritive fluid contain the materials 

 for them, either ready formed, or in a state in which the cell itself can produce 

 them by a process of chemical transformation. Thus, Adipose tissue cannot be 

 formed, unless there be fatty matter in the blood ; the Red Corpuscles cannot 

 be produced without a supply of iron; the Secreting cells of the Kidney require 

 for their development a supply of urinary matter ; and so on. But further, 

 there is evidence that the presence of a particular substance in the nutritive fluid 

 determines the development of the particular kind of cell of which it is the 

 appropriate pabulum, provided that the other requisite conditions are supplied. 

 Thus, an increase of Adipose tissue takes place, when the blood habitually con- 

 tains an unusual amount of fat; an augmentation in the proportion of the Red 

 Corpuscles of the blood may be distinctly observed (especially if they have been 

 previously diminished unduly), when an additional supply of iron is afforded 

 (174); and when one of the kidneys has been removed, or is prevented by 

 disease from performing its normal duty, the other, if it remain healthy, under- 

 goes an extraordinary increase in size, so as to perform the duty of both organs, 

 the augmented development of its secreting structure being here also fairly 



