ABSORBENT CELLS. 



153 



Fig. 32. 



of its birth, through the medium of its umbilical vessels ; the 

 ramifications of which form tufts, that dip down, as it were, into the 

 maternal blood, and receive from it the materials destined to the nutri- 

 tion of the foetus, besides effecting the aeration of the blood of the latter, 

 by exposing it to the more oxygenated blood of the mother. Now around 

 the capillary loop of the foetal tuft, there is a layer of cells, closely re- 

 sembling the absorbent cells of the villi ; and these are enclosed in a cap 

 of basement-membrane, which completes the foetal portion of the tuft, 

 and renders it comparable in all essential respects to the intestinal villus. 

 It is again surrounded, however, by another layer of membrane and of 

 cells, belonging to the maternal system ; the derivation and arrange- 

 ment, of which will be explained hereafter. The maternal cells (6, Fig. 

 32), may be regarded as the first selectors of nutriment from the circu- 



Extrernity of aplacental Tillus: a, external membrane of the Tillus, continuous with the lining mem- 

 brane of the vascular system of the mother ; 6, external cells of the Tillus, belonging to the placental decidua; 

 c, c, germinal centres of the external cells.; d, the space between the maternal and foetal portions of the 

 Tillus; e, the internal membrane of the Tillus, continuous with the external membrane of the chorion; /, 

 the internal cells of the Tillus, belonging to the chorion; g, the loop of umbilical Tessels. 



lating fluid of the parent : the materials, partially prepared by them, are 

 poured into the cavity (d) surrounding the extremity of the tuft ; and 

 from this they are taken up by the foetal cells (/), k which further elabo- 

 rate them, and impart them to the capillary loop (g) of the umbilical 

 vessels. 



246. Thus we see that the several functions of Selection, Absorption, 

 Assimilation, Respiration, Secretion, and Reproduction, are performed 

 by the agency of cells in the Animal as in the Vegetable kingdom, in 

 the complex Human organism, as in the humblest Cryptogamic Plant : 

 the only difference being, that in the latter there is a greater division of 

 labour, different groups of cells being appropriated to different functions, 

 in the general economy, whilst the history of their own processes of 

 nutrition and decay is everywhere essentially the same. Thus we have 

 seen that the Absorbent cells, at the extremities of the intestinal or pla- 

 cental villi, select and draw into themselves, as the materials of their 

 own growth, certain substances in their neighbourhood ; which are still 

 as much external to the tissues of the body, as are the fluids surrounding 

 the roots of plants. Having come to their full term of life, they give 

 up their contents to the absorbent vessels, which carry them into the 

 general current of the circulation, where they are mingled with the fluid 

 previously assimilated, the blood. Whilst passing through the vessels, 

 they are subjected to the action of the various cells (all of which we have 

 seen to be successive phases of the same type) which float in the circu- 

 lating current ; and by these they seem to be gradually assimilated, or 



