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 
