THE VASCULAR SYSTEM 189 



The ventricle (ven.), like that of the Frog, is a muscular sponge 

 with a central cavity, but the sponge is much less dense than in that 

 animal, and the central cavity is comparatively smaller. From the 

 work of Benninghof (1920-1) there would appear to be some order 

 in the seeming chaos of muscle bands composing the sponge of the 

 ventricle, but the questions raised by this author are too special to 

 be entered into here. The blood in many places comes very close to 

 the external surface of the ventricle, and in a freshly killed specimen 

 it is clearly visible within the meshes of the sponge, as the ventricular 

 wall is semi-transparent. The 'central' cavity does not extend any 

 distance caudad of the auriculo-ventricular opening, and is a more or 

 less L-shaped canal leading from that opening to the bulbus cordis. 

 For particulars of the vena cordis see p. 1 9 1 . 



Of the rather complicated structure interposed between the ven- 

 tricle and the aortae, often loosely referred to as the 'truncus arteri- 

 osus', only the distal portion is actually the truncus arteriosus, homo- 

 logous with the ventral aorta of the fish. The proximal portion is the 

 bulbus cordis. 



The bulbus cordis (b.c.) is a long, narrow, slightly twisted tube, 

 cylindrical at the ventricular end and expanding in a horizontal plane 

 to a broad oval distally. It is delimited from the ventricle by the pre- 

 sence of three pocket valves which prevent the reflux of the blood 

 into the ventricle. When closed each forms a cup with the con- 

 vexity towards the ventricle. They are non-muscular and thin-walled, 

 with but few trabeculae stretching across their concavity from the 

 bulbus wall. The dorsal valve is distinctly larger than the other 

 two. The limit between the bulbus cordis and the truncus arteriosus 

 at the distal end of the former is also marked by pocket valves, 

 of which there are four — two dorsal and two ventral. They are 

 also non-muscular but are V-shaped rather than cup-shaped, and 

 are prolonged some distance in a caudal direction along the wall of 

 the bulbus, forming ridges thereon. This is especially pronounced 

 in the case of the right dorsal valve which is contiguous with the so- 

 called spiral-valve — or better septum bulbi — a longitudinal septum 

 partially dividing the cavity of the bulbus into right and left cham- 

 bers. The septum bulbi is much more rudimentary than in the Frog 

 and does not extend any distance anteriorly into the truncus, and 

 it is very doubtful whether it has any influence on the separation of the 

 venous from the arterial blood (cf. p. 1 91 et seq^. It passes across the 

 dorsal wall of the bulbus from left posterior to right anterior. The 

 distal valves frequently have small knobs of hyaline cartilage of vary- 

 ing size embedded in them. They are also much thicker than the 



