464 OF NUTRITION. 



to its power of movement, by affording fixed points for the attachment of its 

 muscles. Others again have coalesced into Vessels, as in plants, for the rapid 

 conveyance of fluids. Others, too, after a similar coalescence, have developed 

 new and remarkable products in the interior of the tubes thus formed ; and 

 become transformed into those Nervous and Muscular tissues, to which nothing 

 analogous is found in Plants, and which are the peculiar instruments of Animal 

 life. Yet still there remains a large number of unchanged Cells scattered 

 through the body ; which perform, as in Plants, the essential part in the func- 

 tions of Nutrition, Reproduction, &c. These, however, could not be held 

 together in their constantly varying relative positions, without some inter- 

 vening substance altogether different from true cellular tissue. It must be 

 capable of resisting tension with considerable firmness and elasticity ; it must 

 admit free movement of the several parts upon one another ; and it must still 

 hold them sufficiently close together to resist any injurious strain upon the 

 delicate vessels, nerves, &c., which pass from one to another, as well as to 

 prevent any permanent displacement. Now all these offices are performed in 

 a remarkably complete degree, by the Jireolar Tissue ( 637); the reason of 

 whose restriction to the Animal kingdom, notwithstanding the purely physical 

 nature of its functions, is thus evident. And as necessity arises, in certain 

 parts, for tissues which shall exercise a still greater power of resistance to 

 tension, and which shall thus communicate motion (as in the case of Tendons), 

 or shall bind together organs that require to be united (as in the case of Liga- 

 ments and Fibrous Membranes), so do we find peculiar tissues developed, that 

 shall serve these purposes in the most effectual manner. Hence these tissues 

 also, although not endowed with any properties that are peculiarly animal, are 

 nevertheless restricted to the Animal Kingdom, as completely as are the Mus- 

 cular and Nervous Tissues, which make up the essential parts of the appara- 

 tus of Animal Life. 



612. That all the Animal tissues are in the first instance developed from 

 Cells, was the doctrine put forth by Schwann, who first attempted to generalize 

 on this subject. By subsequent research, however, it has been shown that 

 this statement was too hasty; and that, although many tissues retain their 

 original cellular type, through the whole of life, and many more are evidently 

 generated from Cells and are subsequently metamorphosed, there are some in 

 which no other cell-agency can be traced than that which was concerned in 

 the preparation of the plastic material. This would appear to be certainly the 

 case, in regard to the very delicate structureless lamella of membrane, now 

 known under the name of Basement or Primary Membrane, which is found 

 (beneath the Epidermis or Epithelium), on all the free surfaces of the body. 

 No vestige of cell-structure can be seen in this membrane ; and it would rather 

 appear to resemble that, of which the walls of the cells are themselves con- 

 stituted.* In some instances it presents a somewhat granular appearance; 

 and is then supposed by Henle to consist of the coalesced nuclei of cells, whose 

 development has been arrested. This, however, is quite hypothetical; and 

 all we can say is, that the Basement membrane is probably formed by the con- 

 solidation of a layer of the plastic element, which may, in certain cases, include 

 a large number of granules that may serve for the development of new cells. 

 Possibly it is in these granular germs, sometimes scattered through the mem- 

 brane, and in other instances collected into certain spots,! that the cells of the 

 superjacent Epithelium or Epidermis take their origin; and if this be the 



* See a Paper by the Author, on the Microscopic Structure of Shells, &c., in the Annals 

 of Natural History, Dec., 1843. The inner layer of the Shells of Mollusca, after treatment 

 with a dilute acid, yields specimens of Basement Membrane, in a form well adapted for 

 examination. 



j- See Goodsir, in Trans, of Roy. Soc. of Edinb., 1842. 



