DEVELOPMENT OF THE CIRCULATORY SYSTEM. 



245 



formed the pleural cavities. The pleurae expand until, instead 

 of lying as minute passages above and behind the pericardium, 

 they come to completely cover it. The abdominal part of the 

 coelom forms the peritoneal cavity ; a small part on each side 

 is shut off in the scrotum, the tunica vaginalis. 



The Pericardium. — In Fig. 202 the relationship of the heart 

 to the pericardium is shown during the 3rd week of foetal life. 

 The heart is tubular ; the dorsal and ventral mesocardia (Fig. 

 193) have already disappeared except at two points, where the 

 heart still remains attached. These two points are (Fig. 202): 



(1) In front where the bulbus arteriosus passes out under the 

 pharynx to divide into right and left ventral aortae from which 

 the right and left visceral aortic arches arise. The mesocardium 

 which binds it here may be named the arterial mesocardium. 



(2) Behind the sinus venosus is imbedded in the mesentery 

 or posterior mesocardium, through which the great veins reach it. 

 The mesocardium which binds it behind may be named the 

 venous mesocardium. 



aorta 



sup. u. cau. 



venous meso-card. 



right pul. ueins 



venous meso-card. 



inf. u. cau 



diaph. 



aorta 



arterial meso-cardium 



pulm. art. 



transverse sinus 

 left pul. veins 



diaph. 



oblique sinus 



Fig. 203. -View of the Interior of the Pericardium showing the attachments of the 

 heart to its dorsal aspect by the Arterial or Venous Mesocardia. 



In Fig. 203 is shown the fixation of the heart within the peri- 

 cardium of the adult. The arterial and venous mesocardia can 

 be recognised, only somewhat altered in position and form. 



(1) The bulbus arteriosus becomes the ascending aorta and 



