BASEMENT OR PRIMARY MEMBRANE. 129 



mary Membrane in contact with cells, which form a more or less conti- 

 nuous layer upon its surface. These cells can only receive their nutri- 

 ment by the imbibition of fluid, through the primary membrane, from 

 the blood brought to its attached surface, by the capillary vessels of the 

 tissue with which it is in relation. Thus in the Skin and Mucous_ 

 membranes, a very copious supply of blood is brought to the attached 

 surface of the primary membrane, by the minutely-distributed capilla- 

 ries which form a large part of the subjacent tissue ; and it is from these 

 that the epidermis and epithelium draw their nourishment, through the 

 primary membrane. In like manner, the ultimate follicles and tubuli 

 of the Glands are surrounded by a copious network of capillaries (Fig. 

 12) ; and it is from these, through the primary membrane, that the cells 

 of these follicles draw their nourishment. Hence this membrane, in 

 every instance, forms a complete septum, on the one hand between the 

 stream of blood in the vessels and the surrounding tissues, since it forms 

 the lining even of the minutest capillaries ; and on the other between 

 the fluids in the interstices of the substance of the true skin, the mucous 

 membranes, &c., and the cells covering their free surfaces. It is evi- 

 dent, therefore, that whilst bounding these tissues and restraining the 

 too-free passage of fluids from their surfaces, it allows the transudation 

 of a sufficient amount for the nutrition of the cells which lie upon it; 

 and, as we shall presently see, these cells frequently pass through all 

 their stages of growth so rapidly, that a very free supply of nutriment 

 must be required by them. Hence, notwithstanding its apparent homo- 

 geneousness, the primary membrane must have a structure which readily 

 admits the passage of fluid. In this respect it corresponds with the 

 membrane, which forms the wall of the cells of both Animal and Vege- 

 table tissues ; for this also appears completely homogeneous and struc- 

 tureless, when seen under its simplest aspect, and yet allows the free 

 passage of fluids from one cell to another. 



208. But it is probable that this membrane performs a much more 

 important office than that of simply limiting the fluids, whilst allowing 

 the requisite transudation. We can scarcely account for the new pro- 

 duction of cells, which (as will presently appear) is continually taking 

 place on its surface, without referring to it as the originator of these 

 cells, that is, as the source of their germs. The new generations of cells 

 cannot here be developed by the reproductive powers of the old ones 

 ( 212) ; since the latter are often completely cast off entire, before they 

 liberate the reproductive granules ; or they undergo changes which 

 evidently unfit them for such a purpose. , Thus in the Epidermis we 

 shall find that they become flattened into dry scales, forming an almost 

 horny layer on the surface of the body ; whilst the new cells are origina- 

 ting beneath, from the surface of the basement-membrane ( 224 and Fig. 

 20). Hence we cannot find any other origin forihese cells, than in the 

 basement-membrane itself; and there seems every probability that the 

 granules, which have been mentioned as being frequently diffused through 

 it, are in reality the germs of cells to be developed from its surface ; 

 whilst the distinct spots are collections of similar granules, each of which 

 may give origin to a large number of such cells, which spring from them 

 as from a centre. We shall presently see that these " germinal centres" 



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