618 MR. c. H. o'donoghue on the 



against the body-wall, while on the left side it is separated from 

 it by the intervention of the CBSophagus. In conjunction with 

 the elongated form of the body, we find that the heart also is 

 long and narrow. 



The Sinus Venosus is situated on the dorsal surface of the 

 heart, and appears as a saccular structure divided into two parts 

 and formed by the swollen extremities of the common jugular 

 veins and the post-caval veins (text-fig. 87). It is hardly dis- 

 tinguishable externally from the right auricle, although internally 

 the two cavities are separated by the bicuspid sinu-auricular valve. 

 The right common jugular vein from the anterior end of the body 

 and the post-caval from the posterior end join together to form 

 the major part of the sinus venosus, which lies on the right of 

 the dorsal surface of the heart. The left common jugular vein 

 runs down along the outer edge of the left auricle and then across 

 in the groove between the left aui-icle and ventricle (text-fig. 87). 

 Its mouth opens into the smaller part of the sinus venosus *, 

 which is partially separated from the major part by a valvular 

 septum. The efficacy of this septum is seen when injecting, for 

 while the right common jugular and post-caval veins may be easily 

 injected from the major part of the sinus, it is almost impossible 

 to inject the left common jugular from it. 



The thin- walled Auricles (text-fig. 86) are unequal in size, the 

 right, of an elongated oval form, being much larger than the left, 

 which is shorter and inore rectangular. Into the right auricle 

 opens the sinus venosus and into the left the single pulmonary 

 vein (text-fig. 87). The opening of the pulmonary vein is not 

 guarded by a valve as Fritsch (16) pointed out, but it seems 

 highly probable, as Sabatier (33) suggested, that during systole 

 a fold of the auricle in this region functions as a valve and so 

 prevents regurgitation. The auricles are completely separated 

 by an imperforate inter-auricular septum which is continued 

 caudally so as to divide the auriculo- ventricular aperture into two. 

 The internal surfaces of the auricles possess a network of raised 

 muscular ridges, the musculi pectinati. 



The Ventricle is somewhat oval in shape, but very asymmetrical. 

 The posterior end forms a bluntly conical apex, and the base, 

 although more or less transverse on the right side, is produced 

 anteriorly on the left side into a conical process, so that the left 

 side of the ventricle is nearly as long again as the right. It is 

 extremely thick-walled, and its cavity contains a large number of 

 muscular trabeculse, some of which interlace in such a way as to 

 form an oblique, incomplete ventricular septum. This partial 

 septum keeps the aerated blood brought in by the left auricle 

 more or less completely separated from the non-aerated blood 

 from the right auricle. Two valves, a right and a left, similar in 

 arrangement to those in Lacertilia, guard the auriculo-ventricular 

 apertures. 



* They do not open separate!}' into the auricle as stated in Rolleston (32). 



