GENERAL SURVEY OF THE FROG 7 



toneum (frequently but less correctly called for brevity peritoneum}. It should 

 be recalled that in man the body cavity is divided by means of a muscular parti- 

 tion, the diaphragm, into two completely separated portions, an anterior thoracic 

 cavity and a posterior abdominal cavity. The lining membrane of the former is 

 then called pleura, and that of the latter, peritoneum, in the correct sense. Since, 

 however, in the lower vertebrates, including the frog, a diaphragm has not yet 

 evolved,, one continuous coelome, or pleuroperitoneal cavity, is present, and its 

 lining membrane is named the pleuroperitoneum. 



The body wall thus consists of three layers: the skin, the muscles with their 

 contained skeleton, and the coelomic lining, or peritoneum, with the subcutaneous 

 lymph spaces lying between the first two layers. 



2. The peritoneum and mesenteries. The peritoneum not only lines the 

 coelome, but forms a close investment of all the viscera, for which purpose it is 

 frequently pulled away from the body wall as a double-walled membrane. That 

 portion of the peritoneum which adheres to the inside of the body wall is called 

 the parietal peritoneum; that which invests the viscera is the visceral peritoneum, 

 or serosa; and that which extends from the body wall to the individual organs 

 or from one organ to another is a mesentery or ligament. In the formation of a 

 mesentery, the peritoneum leaves the body wall, passes over the surface of the 

 organ, and returns to the body wall at the same point from which it left, produ- 

 cing a double-walled membrane between the organ and the body wall. It is thus 

 evident that the peritoneum is everywhere continuous and unbroken, and that 

 the viscera are really outside of the peritoneum, which forms a closed sac into 

 which the viscera appear to be pushed from without. The condition is not really 

 brought about in this way but by the fact that the peritoneum develops later 

 than the viscera and closes over them after they have formed. The visceral 

 peritoneum is so tightly applied to the surface of the viscera that it cannot be 

 separated from them. 



Extreme caution must be used in examining the following mesenteries, 

 especially those in the region of the heart, so as not to destroy them. 



Note the following mesenteries. Lift up the pectoral girdle cautiously and 

 find beneath it a thin-walled sac, the pericardial sac, which contains the heart. 

 Pick up the pericardial sac gently with a forceps and observe that it is separated 

 from the heart by a space, the pericardial cavity, in which the heart moves freely. 

 The heart is in reality, like the other viscera, inclosed in a double sac; the inner 

 one tightly invests the heart, constituting in fact a serosa, or visceral pericardium; 

 the outer sac as already noted is loose and separated from the heart by the peri- 

 cardial cavity, forming a parietal pericardium. The pericardium is therefore a 

 part of the genera lining of the coelome, and the pericardial cavity is a part of 

 the coelomic cavity, from which, however, it has become completely separated 

 during embryonic development, by the formation of the pericardial sac (parietal 

 pericardium) . 



