144 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



round the nipple of pregnant women ; and sometimes large patches of 

 the cutaneous surface, on the lower part of the body especially, become 

 almost as dark as the skin of the Negro. On the other hand, individuals 

 are occasionally seen with an entire deficiency of pigment-cells, or at 

 least of their proper secretion, not merely in the skin, but in the eye ; 

 such are termed Albinoes ; and they are met with as well among the 

 fair, as among the dark races. The absence of colour usually shows 

 itself also in the hair ; which is almost white. 



231. The Epithelium may be designated as a delicate cuticle, covering 

 the free internal surfaces of the body ; and apparently designed, in some 

 instances, simply for their protection ; whilst in other cases, as we shall 

 presently find, it serves purposes of far greater importance. It has long 

 been known that the Epidermis might be traced continuously from the 

 lips to the mucous membrane of the mouth, and thence down the oesopha- 

 gus into the stomach ; and that in the strong muscular stomach or giz- 

 zard of the granivorous birds, it becomes quite a firm horny lining. 

 But it has been only ascertained by the use of the Microscope, that a 

 continuous layer of cells may be traced, not merely along the whole sur- 

 face of the mucous membrane lining the alimentary canal, but likewise 

 along the free surfaces of all other Mucous membranes, with their pro- 

 longations into follicles and glands; as well as on Serous and Synovial 

 membranes, and the lining membrane of the heart, blood-vessels, and 

 absorbents. The Epithelial cells, being always in contact with fluids, 

 do not dry up into scales like those of the Epidermis ; and they differ 

 from them also in regard to the nature of the matter which they secrete 

 in their interior. In this respect, however, the Epithelial cells of dif- 

 ferent parts are unlike one another, fully as much as any of them are 

 unlike the cells of the Epidermis ; for we shall find that all the secretions 

 of the body are the product of the elaboration of Epithelium cells ; 

 and consequently there are as many varieties of endowment, in these 

 important bodies, as there are varieties in the result of their action. 



232. The Epithelium covering the Serous and Synovial membranes, 

 and the lining of the blood-vessels, is composed of flattened polygonal 

 cells (resembling those shown in Fig. 23), lying in apposition with each 

 other, so as to form a kind of pavement ; hence this form is termed pave- 

 ment- or tessellated-epithelium. There is no reason to believe that it 

 possesses any active endowments in these situations ; since it does not 

 appear to be concerned in the elaboration of any peculiar secretion. It 

 has been already pointed out ( 196), that the fluid of serous membranes 

 is separated from the blood by a simple act of mechanical transudation 

 (which often takes place to a great extent after death) ; the walls of the 

 blood-vessels do not appear to be concerned in forming any peculiar 

 secretion ; and the only product of this kind, which indicates any special 

 endowment in the epithelium-cells, is the synovia, which is probably 

 elaborated by the cells covering the vascular fringes of the synovial 

 membrane, formerly mentioned ( 198). The cells draw it from the blood, 

 during the progress of their growth, form it as a secretion within them- 

 selves, and then cast it into the general cavity of the joint (when their 

 term of individual life is ended), either by the rupture or the liquefac- 

 tion of their walls. In other cases, it w r ould seem as if the epithelial 

 cells were not frequently cast off and renewed, but possessed a considera- 



