i644 A MANUAL OF ANATOMY 



septum iransversum, which gives rise to the ventral portion of the 

 diaphragm. The pericardial cavity is also shut off from the two 

 pleural cavities by the formation of the pieuro-pei'icardial membrane. 

 Finally, the pleural cavities are shut off from the peritoneal cavity 

 by the pleuro-periioneal membranes, which give rise to the dorsal 

 part of the diaphragm. 



Demarcation of the Pericardial Cavity. — ^The pericardial cavity is 

 the first of the divisions of the coelom to be differentiated. As 

 stated, it is cut off from the peritoneal cavity by the septum trans- 

 versum, which gives rise ts the ventral portion of the diaphragm. 



Septum Transversum. — ^This is a thick, incomplete lamina of 

 mesoderm, which is situated at the caudal end of the pericardial 

 region on a level with the third and fourth cervical somites. It 

 separates the sinus venosus of the primitive heart within the peri- 

 cardial portion of the coelom or body-cavity from the primitive 

 stomach and intestinal tube, within the peritoneal portion of the 

 coelom. At first it occupies an almost horizontal plane, but, as 

 the heart extends dorsal wards, it gradually assumes an oblique 

 plane, the ventral attachment of the septum descending, so that 

 the direction of the plane is do rso-ventral wards. As just stated, 

 the septum is incomplete, the deficient part being situated dorsally, 

 and its attachments are as follows: Ventrally (anteriorly) it is at- 

 tached tp the ventral wall of the coelom, in which situation it 

 receives the vitelline and umbilical veins on their way to the sinus 

 venosus of the primitive heart. Laterally it is attached to the 

 lateral wall of the coelom on either side (lateral mesocardia), where 

 it receives the corresponding duct of Cuvier, these two ducts being 

 likewise on their way to the sinus venosus. Dorsally the margin of 

 the septum may be divided into three parts — ^mesial and two lateral, 

 right and left. The mesial part faces, and is soon closely connected 

 with, the ventral aspect of the primitive duodenum, at the place 

 where the hepatic diverticulum makes its appearance. The lateral 

 parts of the dorsal margin are free and form the ventral boundaries 

 of the peritoneal openings of the two thoracic (pleural) recesses of 

 the peritoneum. 



As development proceeds, the septum transversum becomes 

 separated into three layers — cephalic, mesial, and caudal. The 

 cephalic or pericardial layer becomes the diaphragmatic portion of 

 the pericardial wall. The middle or diaphragmatic layer gives rise 

 to the ventral portion of the diaphragm. The caudal or hepatic layer 

 becomes separated from the middle or diaphragmatic layer, as the 

 development of the liver from the hepatic diverticulum proceeds 

 within it, and this layer gives rise to the connecting tissue of the 

 liver. 



The deficiency left by the septum transversum dorsally is filled 

 up by the two pieuro-peritoneal membranes, right and left, to be 

 presently described, and in this manner the dorsal part of the dia- 

 phragm is formed. ' '"''[' 



As stated, the septum transveiium is incomplete dorsally. On 



