THE PLACENTA. 



669 



similar to the conditions of the lung alveoli, where the blood is 

 separated from the air with which it interchanges gases by the 

 cells of the capillary wall and of the lung alveolus. 



Though the capillaries of the foetus are in such close relation 

 to the blood of the mother, it must be distinctly understood that 



FIG. 258. 



Antero-posterior section through a gravid uterus and ovum of five weeks 

 (semi-diagrammatic). (Allen Thomson.) a. Anterior wall of uterus, p. Pos- 

 terior wall of uterus, ra. Muscle substance, g. Glandular layer, ss. De- 

 cidua serotina. r. Decidua reflexa. v. Decidua vera. ch. Chorion. u.u. 

 Uterine cavity, c. Cavity of the cervix. 



there is no direct communication between the vessels of the fcetus 

 and those of the mother, and therefore it is not possible to inject 

 the vessels of the mother through those of the fcetus, or vice versa . 



