7.50 



NUTRITION. 



the ribs, for instance, a more or less distinct 

 fibrous appearance may be frequently seen in 

 the intercellular substance ; this is sometimes so 

 faint, that it might be considered as an illusion, 

 occasioned by the manipulation to which the 

 section has been subjected ; but it is often so 

 well-defined, as almost to present the appear- 

 ance of the true fibrous structure. No indica- 

 tion of the direct operation of cells in the 

 developement of these fibres has ever been wit- 

 nessed ; and we can scarcely do otherwise than 

 regard them as produced by the regular ar- 

 rangement and consolidation of the particles of 

 the blastema or plastic element, in virtue of its 

 own inherent powers. 



The production of the simple structureless 

 membranes which exist in various parts of the 

 body must be attributed, we think, to the con- 

 solidation of a thin layer of blastema, rather 

 than to any metamorphosis of cells. The 

 basement or primary membrane which lies 

 beneath the epithelium of the mucous and 

 serous membranes, and of the glandular pro- 

 longations of the former, as well as the mem- 

 brane lining the bloodvessels, and bearing 

 epithelium upon its inner surface, must pro- 

 bably be regarded in this light. It may be 

 questioned, however, whether this is not to be 

 regarded, in most cases at least, as a transitional 

 form, rather than as a permanent structure. 

 We have reason to believe that in many situa- 

 tions (as the lining of the alimentary canal and 

 of its glandular prolongations,) the nuclei con- 

 tained in this membrane must be continually 

 developing themselves into epithelium-cells ; 

 and in some other instances it would seem, that 

 a fibrous structure developes itself from it by a 

 metamorphosis of a different kind. It is not 

 difficult to imagine, that these variations may 

 have their origin in the degree' of plasticity of 

 the element, of which the membrane was origin- 

 ally composed, and in the number of cell-germs 

 which it includes. Considerable differences in 

 the appearance of this primary membrane may 

 be seen, in examining the residua left after dis- 

 solving away the calcareous matter of shells by 

 dilute acid. Putting aside the cellular tissue 

 which certain shells exhibit,* the most general 

 animal basis of each layer is a very delicate 

 membrane, which sometimes appears com- 

 pletely homogeneous, even when viewed with 

 the highest powers of the microscope ; but 

 which in other instances presents a distinctly 

 granular aspect, as if it consisted of a layer of 

 molecules consolidated together by a structure- 

 less cement. These membranous films are in- 

 cluded between strata of calcareous matter, 

 poured out from the surface of the mantle, and 

 thus undergo no change subsequent to their 

 first production. 



We have next to consider the mode in which 

 the tissues, whose form is distinctly cellular, or 

 which can be clearly proved to originate in 

 cells, derive their nutriment from the blood. 

 In the early stage of embryonic developement, 



* See a paper by the author on the Microscopic 

 Structure of Shell, in Annals of Natural History, 

 December, 1843, 



as already stated, the whole fabric is composed 

 of cells which present no recognizable differ- 

 ences amongst themselves, and which yet, by a 

 process of histological transformation, become 

 the elements of the different organs, some of 

 them still retaining the form of cells, whilst 

 others undergo changes which remove them 

 altogether from that category. To the former 

 class belong adipose tissue, pigment-cells, the 

 various kinds of epithelium and epidermis, 

 cartilage-cells, &c. Of the latter, the capillary 

 bloodvessels, and the muscular and nervous 

 tissues, are characteristic examples. Now there 

 would seem much reason to believe, that in the 

 regular process of nutrition, each of these 

 tissues draws from the blood the materials 

 necessary for its reparation and growth, as it 

 does in the earlier stages for its first develope- 

 ment ; and that the function of the blood is 

 confined to the supply of these materials, the 

 germs of the new tissue being supplied by that 

 previously existing. At any rate it may be 

 safely affirmed that no evidence has been ad- 

 duced which renders any other view probable. 

 The self-nutrient power of the tissues is evinced 

 by this fact among others, that in no instance 

 are their ultimate elements penetrated by the 

 capillary bloodvessels. Thus although adipose 

 tissue is traversed by a minute capillary net- 

 work, the fat-cells lie in the meshes of this net- 

 work, and are as independent of it, except as 

 regards the supply of nutrient materials which 

 they derive from it, as if they adhered closely 

 to each other. The muscular fibres and nerve 

 tubes, again, are not penetrated by capillary 

 vessels, but are only surrounded by them. The 

 connection of the cartilage-cells with the ves- 

 sels is still more remote; for the true cellular 

 cartilages are not penetrated by bloodvessels at 

 all (in the healthy state at least), but are 

 nourished by the imbibition of fluids from a 

 plexus of dilated vessels that comes into rela- 

 tion with their external surface. We may infer, 

 therefore, that the bloodvessels are subservient 

 to the act of nutrition only by conveying the 

 nutrient fluid into the neighbourhood where it 

 is required, just as, in the irrigation of a mea- 

 dow, the water is carried in channels over the 

 general surface, but has to find its way by 

 percolation into thespaces between these; and 

 that it is by the materials which they derive 

 from it that the several tissues are enabled to 

 maintain their integrity, by reproducing their 

 structure as fast as it is disintegrated. And it 

 may not be unreasonable to infer that, in the 

 very act of the death and disintegration of the 

 parent structure, the germs of the new struc- 

 tures destined to replace it are set free, as hap- 

 pens in the reproduction of the simple cellu- 

 lar plants. 



It may be doubted, however, whether the 

 same holds good in regard to newly-forming 

 parts, or with respect to the epithelium cells, 

 which are formed on the free surface of the 

 basement membrane, and which are cast off 

 without reproducing themselves. These last 

 seem to originate in germs contained in the 

 subjacent membrane, and a continual supply of 

 such germs must therefore be required. It can 



