OF ORGANIZED STRUCTURES IN GENERAL. 21 



cal properties, as we never encounter amongst Mineral bodies. In the 

 latter, solidity or hardness may be looked upon as the characteristic con- 

 dition ; whilst in Organized structures, softness (resulting from the large 

 proportion of fluid components) may be considered the distinctive quality, 

 being most obvious in the parts that are most actively concerned in vital 

 operations. This softness is evidently connected with the roundnes*o 

 form characteristic of Organized fabrics, which is most evident when 

 the tissues contain the greatest proportion of fluid ; whilst the plane 

 surfaces and angular contours of Mineral bodies are evidently due to 

 the mode in which the solid particles are aggregated together, without 

 any intervening spaces. 



8. The greatest solidity exhibited by Organized fabrics, is found where 

 it is desired to impart to them the simple physical property of resistance ; 

 and this is attained by the deposition of solid particles, often of a mineral 

 character, in tissues that were originally soft and yielding. It is in this 

 manner that the almost jelly-like substance, in which all the organs of 

 animals originate, becomes condensed into cartilage, and that the carti- 

 lage is afterwards converted into bone ; it is in the same manner, also, 

 that the stones of fruit, and the heart-wood of timber-trees, are formed 

 out of softer tissues. But, as we shall hereafter see, this kind of con- 

 version, whilst it renders the tissue more solid and durable, cuts it off 

 from any active participation in the vital operations ; and thence reduces 

 it to a state much more nearly analogous to that of mineral bodies. 

 This resemblance is rendered more close by the fact, that the earthy 

 deposits frequently retain a distinctly crystalline condition ; so that, 

 when they are present in large proportion, they impart a more or less 

 crystalline aspect to the mass, and especially a crystalline mode of frac- 

 ture, which is evident enough in many shells. It must not be hence 

 concluded, however, that such substances are of an inorganic nature ; 

 all that is shown by their crystalline structure being, that the animal 

 basis exists in comparatively small amount, and that the mode in which 

 the mineral matter was deposited has not interfered with its crystalline 

 aggregation. 



9. It is not to be disputed that a certain degree of homogeneity is 

 apparently to be found in the minutest elements, into which certain 

 Organized tissues are to be resolved. Thus, in the membranes which 

 form the walls of Animal and Vegetable cells, the highest powers of the 

 microscope fail in detecting any such distinction of fluid and solid com- 

 ponents, as that which has been described as characteristic of organized 

 structures. Nevertheless it is indubitable that such distinct components 

 must exist ; and this especially from the properties of these membranes 

 in regard to water. For it is one of the most remarkable facts in the 

 whole range of science, that a membrane, in which not the slightest 

 appearance of a pore can be discovered under the highest powers of the 

 microscope, should be capable of allowing water to pass through it ; and 

 that, too, with no inconsiderable rapidity. The change which these 

 membranes undergo in drying, is another proof that they are not so 

 homogeneous as they appear, and that water is an element of their struc- 

 ture, not merely chemically, but mechanically. The same may be said 

 in regard to the fibres, which form the apparently ultimate elements of 



