98 Anatomy of the Rabbit. 



there are two complete uteri, and as an organ the whole structure 

 represents the stage of uterus duplex (Fig. 54 A). A partly 

 fused condition existing in some mammals, for example sheep, is 

 known as uterus bicornis (Fig. 54 B), while the completely fused 

 condition in man is known as uterus simplex (Fig. 54 C). It is 

 characterized by the independent opening of the two uterine tubes 

 into a single uterine cavity. The successive stages of coalescence 

 are doubtless associated with progressive reduction of the number 

 of young, the success of the species being determined by greater 

 perfection of the placental apparatus. 



THE SEROUS CAVITIES. 



The organs collectively described as visceral are those associated 

 with the serous cavities. They belong to several systems, but 

 present the common feature of being projected into the membranous 

 linings of these cavities so that they are more or less completely 

 invested by them. 



The serous sacs are extensive body-spaces, derivatives of a 

 primary body cavity or coelom. They are usually considered as 

 containing the visceral organs, but the condition is more accurately, 

 described as one in which the visceral organs encroach, chiefly from 

 a dorsal position, on the enclosing membranes. The latter are thus 

 divided into two portions, one of which is distributed as a parietal 

 or peripheral layer, forming the enclosure of the sac, while the other 

 is disposed as a visceral layer on the surface of the visceral organs. 

 The serous sacs are enclosed by thin, moist, serous membranes, 

 consisting chiefly of mesothelium, which give to the visceral organs 

 their characteristic appearance. 



In lower vertebrates, where the diaphragm is absent or imper- 

 fectly developed, the coelom is divided into two chief portions — the 

 pericardial cavity, enclosing the heart, and the pleuroperitoneal 

 cavity, lodging the remaining visceral organs, including in terres- 

 trial vertebrates the .lungs. In the mammalia the pleuroperitoneal 

 cavity is completely divided into two portions by the diaphragm, 

 the smaller pleural portion being again divided into right and left 

 pleural cavities through the presence of certain structures filling 

 the median portion of the thorax. There are thus recognizable in a 



