42 BASEMENT MEMBRANE I CELLS. 



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 j 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 

 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 



Fig. 4._N UCLE A TED CELLS; a a, nuclei. ^.^ formed Q a stmcture . 



less membrane, and having its cavity filled with fluid of some 

 kind. In some part of its interior, most commonly adhering 



