‘ 
42 BASEMENT MEMBRANE :—-OELLS. 
front of the eye be so injured by accident or disease, that light 
cannot pass through it to make its impression on the nerve, 
that nerve, being thrown into disuse, will gradually degenerate 
into fibrous tissue. Moreover, this change may take place as 
a part of the regular actions of life; for there are certain 
organs in the young animal previous to birth, which are not 
required afterwards ; and these degenerate in like manner, 
gradually wasting away, and leaving only traces behind them, 
—tubes shrivelling into fibrous ligaments, and glandular 
structures remaining only as areolar tissue. 
31. Along every free surface of the body, both external 
_and internal, is spread out a delicate structureless layer, which 
is termed the Basement or Primary Membrane. This forms 
the outer layer of the True Skin, lying between it and the 
Epidermis or scarf-skin (§ 37); in the same manner it 
underlies the Epithelial layer of the Mucous membranes 
which line the open cavities of the body (§ 39), and of the 
Serous membranes which line its closed cavities (§ 43); 
and it occupies the same position in the walls of the blood- 
vessels, gland-ducts, and other tubes. It is difficult to sepa- 
rate it, in any of these parts, from the tissues with which it 
is in contact ; and its characters may be well studied by dis- 
solving the calcareous part of an oyster or mussel-shell in 
dilute acid, when it will be found that layers of a thin trans- 
parent membrane are left, which have been thrown off at 
each act of shell-formation, from the surface of the mantle. 
This elementary membrane, like that which forms the walls’ 
of cells (§ 32), is remarkable for the readiness with which 
it is permeated by fluid, al- 
though no visible pores can 
i be seen in it. 
32. A considerable part of 
the fabric of even the highest 
Animal is formed, like the 
entire organism of the Plant, 
of Cells, either unchanged or 
in some way metamorphosed. | 
A cell is a minute bag or’ 
vesicle, formed of a structure- 
less membrane, and having its cavity filled with fluid of some 
kind. In some part of its interior, most commonly adhering 
Fig. 4.—NucLeatep Cex1s; a a, nuclei. 
