STRUCTURE OF HUMAN PLACENTA. 44*7 



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 s.een when it passes from one loop or coil 

 of the bloodvessel of the villus on to another. 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 undergone 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. O/ the Bloodvessels of the Tufts. 



Within the internal membrane, and imbedded in structures 

 to be afterwards described, are situated the bloodvessels 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 relation holds. In others of the primary- 

 branches, and in all the villi, the vessel retains the same mean 

 diameter throughout. This species of bloodvessel, although 

 it cannot be considered as either artery or vein, cannot never- 

 theless 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, containing four or six 

 blood-discs abreast. It is also peculiar in exhibiting sudden 

 constrictions and dilatations, like an intestine. 



