750 
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 virtuevof 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. 
e have next to consider the mode in which 
the tissues, whose form is distinctly celfular, 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 
Stracture of Shell, in Annals of Natural History, 
December, 1843. 
NUTRITION. 
as already stated, the whole fabric is com 
of cells which present no recognizable 
ences amongst themselves, and which yet, by a 
process of histological transformation, 
the elements of the different organs,—some of 
them still retaining the form of cells, ilst 
others undergo changes which remove them 
altogether from that category. To the former 
class belong adipose tissue, pigment-cells, the 
— kinds of epithelium and epi 
cartilage-cells, &c. Of the latter, the capillary 
bloodvessels, and the muscular i eee 
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 cee 
ment; and that the iartien 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 evinces 
by this fact among others,—that in no instan 
are their ultimate elements 
capillary bloodvessels. Thus although adi 
tissue is traversed by a minute capillary 
work, the fat-cells lie in the meshes of this 
work, and are as independent of it, excep 
regards the supply of nutrient materials whic 
they derive from it, as if they adhered closel 
to each other. The muscular fibres and ne 
tubes, again, are not penetrated by capille 
vessels, but are only surrounded by 2 
connection of the cartilage-cells with the 1 
sels is still more remote; for the true cellu 
cartilages are not penetrated by bloodvessels- 
all (in the healthy state at least), but 
nourished by the imbibition of fluids fron 
plexus of dilated vessels that comes into m 
tion with their external surface. We may ini 
therefore, that the bloodvessels are sub 
to the act of nutrition only by conveyin 
nutrient fluid into the neighbourhood wh 
is required,—justas, in the irrigation of an 
dow, the water is carried in channels over 
general surface, but has to find its wa 
percolation into thespaces between these 
that it is by the materials which they 
from it that the several tissues are enable 
maintain their integrity, by reproducing 
structure as fast as it is disintegrated. A 
may not be unreasonable to infer that, i 
very act of the death and disintegration 
parent structure, the germs of the new 
tures destined to replace it are set free, ¢ 
ie in the reproduction® of the simple 
ar plants. -* 
t may be doubted, however, whe 
; 
: 
0 
same holds good in regard to newly- 
parts, or with res to the epitheliun 
which are formed on the free surfa 
basement membrane, and which 
without reproducing themselves. 
seem to originate in germs contained 
subjacent membrane, and a continual su 
such germs must therefore be required. — 
