152 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



Fig. 47. 



manent digestive cavity, and are raised into folds or villi, as the contents of 

 the yolk-bag are diminished. Now the ends of the vessels are separated 

 from the fluid contents of the yolk-bag; by a layer of cells, which is filled 

 with matter of a yellow-colour; and which seems to have for its office, to 

 select and prepare the materials supplied by the yolk, for being received into 

 the absorbent vessels. In like manner, the embryo of the Mammal is nou- 

 rished, up to the time of its birth, through the medium of its umbilical ves- 

 sels ; the ramifications of which form tufts, that dip down (as it were) into 

 the maternal blood, and receive from it the materials destined for the nutrition 

 of the fetus ; besides effecting the aeration of the blood of the latter, by 

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

 capillary loop of the fetal tuft there is a layer of cells, closely resembling the 

 absorbent cells of the villi ; and these are inclosed in a cap of basement-mem- 

 brane, which completes the fetal 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 sys- 

 tem ; the derivation of which will be explained hereafter (Chap. XVII). 



183. The cells which make up the parenchyma of the Liver in the higher 

 animals, seem to be developed under conditions somewhat similar. In the 

 Invertebrata, the Liver is constructed upon the type of the glands in general ; 

 its secreting cells being developed as an epithelium upon the inner wall of the 

 hepatic ducts. This does not appear to be the case, however, in Man and the 

 Mammalia : the substance of whose liver is made up of an aggregation of 

 cells, which lie so far as can be ascertained upon the 

 outside of the terminal ramifications of the hepatic ducts. 

 That these cells are the efficient instruments in the secre- 

 ting process, is evident from the nature of their contents, 

 which consist of biliary matter with oil globules. Their 

 diameter is usually from 1-1 500th to 1 -200th of an inch ; 

 and they generally contain a very distinct nucleus. Their 

 connexion with the secreting process is further marked by 

 the fact, that, in some instances in which the bile has not 

 been eliminated, and death has been the result, Microscopic 

 examination has proved that the hepatic cells were either 

 very imperfectly formed or were almost entirely deficient. 

 Further, in cases of Fatty Liver, the cells have been found to contain an un- 

 usual amount of Adipose matter. 



184. The Fat-cells, of which Adipose tissue 

 is composed, also permanently exhibit the original 

 type of structure in its simplest form. This tis- 

 sue is usually diffused over the whole body, filling 

 up interstices, and forming a kind of pad or 

 cushion for the support of moveable parts. Even 

 in cases of great emaciation, some Fat is always 

 left ; especially at the base of the heart, around 

 the origin of the large vessels ; in the orbit of the 

 eye ; in the neighbourhood of the kidney ; in the 

 interior of the bones ; and within the spinal ca- 

 nal, between the periosteum and the dura mater. 

 The Fat Cells are usually spherical or spheroidal ; 

 Fat vesicles, assuming the poly- sometimes, however, when closely pressed toge- 

 hedrai form from pressure against ther without the intervention of any intercellular 



one another. The capillary ves- gubstance? they become polyhedral. The nucleus 



sels are not represented. From . ,f . . , , 



the omentnm; magnified about 300 1S 11Ot alwa y S . tO be distinguished ; perhaps in 



Secreting Cells of 

 Human Liver; a, nu- 

 cleus; fc, nucleus ; c, 

 oil-particles. 



[Fig. 48. 



diameters.] 



consequence of its having passed to the interior of 



