602 OF REPRODUCTION. 



increased in vascularity, within a short time after conception ; and when the 

 inner surface of a newly-impregnated Uteriis is examined with a low magnify- 

 ing power, the orifices of its tubes are very distinctly seen, being lined with a 

 white epithelium. The blood-vessels form a very minute network, which 

 extends in loops from the subjacent portion of the membrane (as seen in Fig. 

 17, Plate I.). Of the orifices of the glandular follicles, some afterwards become 

 widened and enlarged for the reception of the foetal villi. The thickness of 

 the Decidua, when fully formed, is from one to three lines ; its inner surface 

 is smooth ; whilst that in connection with the Uterus is rough, in consequence 

 of the varying length of the tubes, and of the vascular connections of the two 

 structures. It has not yet been explained how the Decidua is formed con- 

 tinuously over the upper orifice of the Cervix Uteri, and over the orifices of 

 the Fallopian tubes, as is frequently, though by no means uniformly, the case ; 

 and it seems as if a new production must there take place. The formation of 

 the Uterine Decidua occurs whether the Ovum reach the Uterus or not ; it 

 being probably invariable* in cases of extra-uterine pregnancy, even though a 

 Decidua is formed around the Ovum in the place of its lodgment. Besides 

 the Decidua lining the Uterus, however, another membrane, continuous with 

 this, furnishes a proper envelop to the Ovum ; and this has been termed the 

 Decidua reflexa. The formation of this is usually explained, in conformity 

 with the account of Dr. W. Hunter, after the following manner. The Ovum, 

 on passing from the Fallopian tube into the Uterus, pushes before it a portion 

 of the Decidua Vera, as represented in Fig. 181 ; and this portion is gradually 

 extended, by the subsequent growth of the Ovum, so as at last to surround it 

 completely. If this were precisely the case, however, the structure of the 

 two membranes ought to be the same, which it is not ; for, according to the 

 observations of Dr. Sharpey,t the Decidua Reflexa is destitute, in great part 

 of its surface, of the small orifices which characterize the Vera ; and these are 

 confined chiefly, though not entirely, to a zone of the membrane surrounding 

 the angle of reflexion, that is, to the part next to the Decidua Vera. It would 

 seem more probable, therefore, that the Decidua Reflexa is almost entirely a 

 new production, the growth of which is simultaneous with the enlargement of 

 the ovum ; and that the Decidua Vera has no more share in its formation than as 

 supplying, through its vessels, the necessary materials. As the ovum increases 

 in size, the Decidua Reflexa w^hich covers it, comes into contact with the Decidua 

 Vera, which lines the Uterus; and the fluid that previously filled the cavity 

 disappears by absorption ; this usually happens during the third month. After 

 this period, it is difficult and frequently impossible, to distinguish the two 

 layers ; and even in aborted ova of an earlier age, the Decidua Reflexa is not 

 always to be found, on a careful examination ; so that its very existence has 

 been denied by some. At one part of its surface, the Ovum is covered neither 

 by the Decidua vera, nor by the Decidua reflexa ; this is where the former 

 was originally detached from the wall of the Uterus, by the Ovum, and where 

 it becomes continuous with the latter. It is at this point that the Placenta is 

 subsequently formed. The deficiency is supplied, however, by a new pro- 

 duction, very analogous in structure to the Decidua Reflexa, and continuous 

 with the reflected fold of the Decidua Vera ; this is termed (from its formation 

 being supposed to take place at a later period) the Decidua Serotina (Fig. 

 186/). 



* The doctrine of the formation of the Decidua, here adopted on the authority of the 

 two accomplished Anatomists mentioned above, tends to reconcile the contradictory ob- 

 servations which have been recorded on this interesting point ; for in those cases in which 

 nothing but an increase of thickness and sponginess in the Mucous membrane of the 

 Uterus was observable, the very change was in progress, in which the formation of the 

 Decidua consists. 



f Loc. cit. 



