TISSUES OF THE BODY. 



155 



we may perhaps regard epithelium as the regulator of these processes 

 spread over the parts where they take place. 



As a purely cellular tissue untraversed by blood-vessels, epithelium 

 presents to us many sides of cell-life in the most beautiful manner, such, 

 as multiplication, growth, and change of form. That the whole vegetation 

 of the epithelial cells is dependent upon the vessels of their connective- 

 tissue substrata is easy to conceive, though we meet with epithclia on 

 non-vascular portions of the body, as for instance on the cornea and 

 capsule of the lens. But of the direction of the interchange of matter 

 in our tissue we know nothing, either of that in ordinary epithelium, or 

 the modified forms of it, where in the interior of the cells a formation 

 of melanin and other pigments takes place. That this alteration of 

 material in laminated epithelium is only undertaken with any degree ol 

 energy by the younger cells which still possess soft contents, is more- 

 over not difficult to understand. It is also probable, on the other hand, 

 that this interchange of material has ceased entirely in the more superfi- 

 cial scales of laminated epithelia which have undergone horny metamor- 

 phosis : in these also decomposition commences very late. 



Then the cylinder epithelial cells of the small intestine are made the 

 media for much transfer of matter, and moreover, not in their own 

 egotistical interest, and for their own special support : through them, 

 namely, the absorption of fats with the other constituents of the chyle 

 takes place. Here again we are reminded of many kinds of gland- 

 cells. 



Attention has likewise been directed within the last few years to the 

 penetration into the interior of epithelial cells of minute coloured par- 

 ticles, which had been introduced into the circulation of the lymph 

 and blood, and, indeed, of red blood-corpuscles also (1). These we may 

 observe in the goblet-cells of the small intestine and in ciliated cells. 



We are obliged for the present to explain the fact of the occurrence of 

 mucous and pus corpuscles, as well as of contractile elements, in the 

 interior of cylinder and pavement epithelium (fig. 151) in the same manner 

 ( 56). It is manifest that a penetration of small bodies into open 

 goblet-cells might take place without 

 difficulty. But, besides, we find these 

 lymphoid- cells within the epithelial 

 stratum, between the columnar ele- 

 ments of the intestine, engaged in mi- 

 grating from the connective-tissue of 

 the mucous membrane into the lumen 

 of the tube. 



Epithelium may, in general, be set 

 down as a tissue capable of under- 

 going no further development. No 

 doubt from the earliest rudiments, from 

 the cells of the corneous and intestinal 

 glandular la} v er, many other tissues, 

 and some of them of high dignity, 

 take their origin in the construction 

 of the embryonic body, as we shall see in some of the succeeding chapters. 

 But not so in the mature body : its epithelial cells are only able to 

 reproduce similar structures, and not other elements, such as, for instance, 

 fat-cells or connective-tissue corpuscles. 



Fig. 151. Occurrence of mucous and pus- 

 coipuscles in the interior of epithelial 

 cells, a-d, cylinder cells of a biliary 

 duct; e, free pus-corpuscles; /, ciliary cehs 

 of the respiratory tract; g, flat ceiis from 

 the urinary passages. 



