52 THE STRUCTURE OF THE HUMAN PLACENTA. 



4. Of the Internal Membrane of the Villus. 



When a villus, under gentle compression, is viewed by trans- 

 mitted light, there is perceived under the structures already de- 

 scribed, and immediately bounding the blood-vessels, and other 

 parts to be afterwards examined, a membrane finer and more 

 transparent than the external membrane, but strong and firm in 

 its texture. This membrane is most distinctly seen when it passes 

 from one loop or coil of the blood-vessel of the villus on to an- 

 other. It separates very easily from the internal surface of the 

 layer of external cells. I am not disposed to believe that it is 

 attached to this layer, but am of opinion that the spaces which 

 frequently exist between them, even in villi which have under- 

 gone no violence, are due to the presence of a fluid matter, the 

 nature of which will be afterwards considered. Be this as it may, 

 pressure very easily separates this membrane from the external 

 cells, the latter invariably remaining attached to the external 

 membrane, the former continuing in every instance closely rolled 

 round the internal structures of the villus, and following them in 

 all their changes of position. 



5. Of the Blood-vessels of the Tufts. 



Within the internal membrane, and imbedded in structures to 

 be afterw r ards described, are situated the blood-vessels of the tuft. 

 These vessels are branches of the umbilical arteries and veins. 



In the trunk of the tuft, the artery gradually diminishes and 

 the vein increases in size. In some of the primary branches the 

 same rotation holds. In others of the primary branches, and in 

 all the villi, the vessel retains the same mean diameter through- 

 out. This species of blood-vessel, although it cannot be consi- 

 dered as either artery or vein, cannot nevertheless be denominated 

 in precise anatomical language, a capillary. It differs from artery 

 and vein in retaining throughout the same mean diameter ; and 

 from the capillary, properly so called, in its greater calibre, con- 



