PERITONEAL RELATIONS OF THE MAMMALIAN OVARY. 43 



sac wall was thicker and more closely applied to the surface of the 

 ovary ; the cavity of the sac was less distinct, and the fimbriated 

 orifice of the Fallopian tube lay close along the inner face of the 

 ovary. 



Thus the rat and the mouse present us with a fifth type or class 

 of peritoneal ovarian sac, namely, a sac the cavity of which is 

 completely shut off from the abdominal cavity, and whose only 

 communication is with the uterus, through the Fallopian tube. 



In most mammals the ovary is placed in the abdominal cavity, 

 or in a pouch of peritoneum widely open to the abdominal cavity, 

 and the ova, as they escape from the ruptured follicles, are swept 

 into the open extremity of the Fallopian tube. In the rat and 

 mouse, however, the ovary is placed in a sac, the cavity of which is 

 undoubtedly, originally, a portion of the abdominal cavity ; but in 

 the adult animal, indeed, in the case of the rat and mouse, 

 from the period of birth the cavity of the ovarian sac is, as before 

 stated, completely shut off from any communication with the 

 abdominal cavity. The ova pass from the ovary into the sac, and 

 thence along the Fallopian tube to the uterus. They cannot enter 

 the body cavity, being shut off from it by the sac wall. 



It seems impossible to arrive at any satisfactory conclusion 

 concerning the function of this ovarian sac. In its more or less 

 complete forms it evidently prevents or greatly diminishes the 

 possibility of the shed ova passing into the general cavity of the 

 abdomen, and consequently it precludes or lessens the chance of 

 occurrence of abdominal pregnancy ; at the same time it ensures 

 the more rapid passage of the ova into the Fallopian tube. Under 

 these circumstances it might be thought that a well-developed sac 

 would be met with in all those animals in whose case it is necessary, 

 for the preservation of the species, that all the mature ova should be 

 fertilised. Such animals would arrange themselves in at least two 

 classes — 



1. Animals in which but few ova reach maturity. 



2. Animals whose life is short and precarious. 



There seems, however, to be no proof that this is the case. Un- 

 doubtedly the sac is present in its most complete form in rats and 

 mice, both necessarily prolific animals, but it is present in its simplest 

 form in the rabbit and cat, both of which animals produce many 



