The Serous Cavities. 49 



from one cavity to the other. It is also observable in this animal that 

 the gubernaculum is represented in the adult as a short thick cord 

 connecting the end of the testis with the wall of the enclosing sac (Fig. 26). 

 In the female the round ligament of the uterus is a structure similar 

 in a general way to the gubernaculum, and in the rabbit will be seen to 

 be inserted in a depression of the body-wall resembling both in form and 

 position a rudimentary vaginal process. The ligament is continued, 

 however, beyond this point, ending in the wall of the urinogenital aper- 

 ture. The presence of this structure is an indication that the ovary, 

 as well as testis, is subject to change in position. In the adult condition, 

 however, the ovary occupies approximately an original position on the 

 dorsal wall of the abdominal cavity ; and it will be observed in the rabbit 

 that the position of the structure is one also defined by the points of 

 origin of the spermatic arteries and veins of both sexes, with the ex- 

 ception, however, of the left spermatic vein in the male. 



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 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 imperfectly 

 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 terrestrial 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 mammal four large serous spaces, 

 namely, the pericardial, peritoneal, and paired pleural cavities. 



The pericardial cavity, the smallest of these spaces, is situated 

 between the paired pleural cavities. Its enclosing membrane, the peri- 

 cardium, forms a capacious sac for the heart, and is reflected directly 

 over the surface of the latter as a thin membrane, the epicardium. 



The pleural cavities are those lodging the lungs, the latter being pro- 

 jected into them from a medial position. The lining membrane or pleura 



