234 



AN INTRODUCTION TO ZOOLOGY 



of the system. It lies in a triangular space, the pericardial cavity, 

 situated behind and beneath the last pair of gill pouches and so 

 occupies a position underneath the hinder end of the pharynx. 

 This is the primitive place for the heart, much further forward than 



in Rana, and indeed in 

 / what would be the neck 



region of the higher animals. 

 In the embryo the peri- 

 cardial and peritoneal 

 cavities are in open com- 

 A.Ca munication with one 

 another, but they become 

 separated at quite an 

 -V early date by the forma- 

 tion of a septum. This 

 septum completely cuts off 

 the two cavities, and hence, 

 as it forms the posterior 

 limit of the heart chamber 

 and the anterior wall of 

 the abdominal coelom, it 

 is termed the pericardio- 

 peritoneal septum. Before 

 the adult condition is 

 reached a secondary tubular 



outgrowth, the pericardio-peritoneal canal, grows backwards from 

 the postero-dorsal corner of the pericardial space, putting the two 

 cavities in communication with one another again. The posterior 

 part of the floor of the pericardial cavity is formed by the median 

 extension of the coracoid bar and its roof by the basibranchial 

 cartilage. The cavity itself is lined by peritoneum, which is also 

 reflected to form a smooth visceral layer that closely invests the 

 heart and is known as the pericardium. 



The heart itself is composed of four chambers and may be con- 

 sidered as starting at the hinder end of the pericardial cavity by the 

 running together of two large veins, the ductus Cuvieri, from the 

 postero-lateral corners of the cavity to form the sinus venosus. 

 This, the first part of the heart, is a large thin- walled tubular sac 

 lying transversely to the long axis of the body, continuous laterally 

 with the ductus and attached to the pericardio-peritoneal septum 

 by its posterior border. It opens by a single median aperture into 

 the postero-dorsal border of the single large auricle or atrium, the 

 opening being guarded by a membranous sinu-auricular valve. The 

 auricle is a triangular muscular sac with its apex directed forward, 



PCo. 



FIG. 74. -Lateral view of the heart to show 

 the arrangement of the coronary veins. 

 After O'Donoghue. 



A.Co., anterior coronary vein ; Au., auricle ; Co., 

 conus arteries us ; O., opening of ductus Cuvieri; P.Co., 

 posterior coronary vein ; S.V., sinus venosus ; V., 

 ventricle. 



