22 NATUBE AND OBJECTS OF PHYSIOLOGICAL SCIENCE. 



the simple fibrous tissues in Animals, and which are also met with in the 

 interior of certain cells and vessels in Plants. These fibres would appear 

 to be of perfectly simple structure ; yet we know from the loss of fluid, 

 and the change of properties which they undergo in drying, that water 

 must have formed part of their substance. It may be remarked, how- 

 ever, in regard to both these elementary forms of Organized tissue, that 

 the simplicity of their function is in complete conformity with the appa- 

 rent homogeneousness of their structure ; for the cell-membrane is chiefly 

 destined to act, like the porous septum in certain forms of the voltaic 

 battery, as a boundary-wall to the contained fluid, without altogether 

 interfering with its passage elsewhere ; the forces which produce its 

 imbibition or expulsion being probably situated, not in this pervious wall, 

 but in the cavity which it bounds. And, in the same manner, the func- 

 tion of the fibrous tissues, to which allusion was just now made, is of an 

 entirely physical character ; being simply to resist strain or pressure, 

 and yet to allow of a certain degree of yielding by their elasticity. 



10. In all cases in which active vital operations are going on, we can 

 make a very obvious distinction of the structures subservient to them, 

 into liquid and solid parts ; and it is, indeed, by the continual reaction 

 which is taking place between these, that the fabric is maintained in its 

 normal condition. For, as we shall hereafter see, it is liable to a constant 

 decomposition or separation into its ultimate elements : and it is conse- 

 quently necessary that the matters which have undergone that disinte- 

 gration should be carried off, and that they should be replaced by new 

 particles. These processes of removal and replacement, with the various 

 actions subservient to them, make up a large proportion of the life of 

 all Organized beings. Now as all the alimentary matter must be reduced 

 to the liquid form, in order that it may be conveyed to the situations in 

 which it is required, and as all the decomposed or disintegrated matter 

 must be reduced to the same form in order to be carried off, the inter- 

 mingling or mutual penetration of solids and liquids in the minutest parts 

 of the body is at once accounted for. We shall hereafter see that a cell, 

 or closed vesicle, formed of a membranous wall, and containing fluid, 

 may be regarded as the simplest form of a living body, and the simplest 

 independent part or instrument of the more complex fabrics (30). 



11. Organized structures are further distinguished from Inorganic 

 masses, by the peculiarity of their chemical constitution. This pecu- 

 liarity does not consist, however, in the presence of any elementary sub- 

 stances which are not found elsewhere ; for all the elements, of which 

 organized bodies are composed, exist abundantly in the world around. 

 It might have been supposed that beings endowed with such remarkable 

 powers as those of Animals and Plants, powers which depend, as we 

 shall hereafter see, upon the exercise of properties to which we find 

 nothing analogous in the Mineral world, would have had an entirely 

 different material constitution ; but a little reflection will show, that the 

 identity of the ultimate elements of Organized structures with those of 

 the Inorganic world, is a necessary consequence of the mode in which 

 the former are built up. For that which the parent communicates, in 

 giving origin to a new being, is not so much the structure itself, as the 

 power of forming that structure from the surrounding elements ; and it 





