404 MUCOUS MEMBRANES. 



chambers of the mucous tract (the buccal cavityj pliarynx and 

 oesophagus, conjunctiva, vulva, preputial sac, bladder and ureters) 

 are lined only by the mucous layer of the cuticle, the so-called 

 laminated pavement-epithelium. This laminated pavement- 

 epithelium consists, like the rete Malpighii, of a single layer of 

 small, columnar cells, and a stratum of variable depth of larger 

 pavement cells, which grow flatter as they approach the free 

 surface, where they are finally shed. 



As reo;ards the meanino; of this diminution in thickness of 

 the epithelial layer in the vestibular portions of the vegetative 

 tract, there cannot be two opinions. It is the first step towards an 

 increased facility of osmotic interchange between the fluids and 

 gases contained in the cavity, and those contained in the blood. 

 Wherever such interchange is more active, wherever absorption 

 or secretion are going on, wherever it lies at the root of the 

 general nutrition of the organism, the pavement cells disappear 

 entirely, and the columnar cells alone remain. Thus the ali- 

 mentary canal from the cardia to the anus, the respirator}- 

 passages, the female generative organs from the external o^ 

 inwards, are lined with columnar epithelium. Its cells are 

 larger than the columnar elements of the rete ; the}- exhibit 

 manifold varieties of external form, in accordance with the 

 functional necessities of particular regions ; inasmuch, however, 

 as they are planted immediately upon the connective tissue, and 

 as between their bases only a few reserve cells, to replace those 

 which are shed, are here and there apparent — cells which might 

 certainly be demonstrated in the rete Malpighii also — I cannot 

 see any objection to their being viewed as the anatomical equi- 

 valent of the columnar cells of the rete, notwithstanding any 

 modifications of size and form which they may present. 



Like the epithelium, the proper substance of the mucous 

 membrane undergoes adaptation to the special functions of the 

 individual portions of the tract. In parts devoted merely to the 

 transmission and storage of their contents — in the oesoj^hagus, 

 biliary and urinary passages, vagina, &c., we find an equable 

 layer of tough fibrill^e of connective tissue, terminating in a 

 smooth and even surface underneath the epithelium, while on 

 the other side it is uninterruptedly continuous with the bundles 

 of the lax submucous connective tissue. Its texture is very 

 different where the tract is devoted to absorption or secretion. 



