THE VASCULAR SYSTEM 331 



pump, and forms the heart. The veins from the liver and the 

 ductus Cuvieri are received by a sinus venosus, which in turn 

 leads into a thin-walled auricle. The latter passes the blood 

 on to the thick-walled muscular ventricle, by which it is 

 propelled into the anterior portion of the subintestinal vessel 

 which is called the ventral aorta. The arteries are surrounded 

 by smooth muscle, but the musculature of the heart is peculiar 

 and unique in that it shows a number of cross-striations and 

 its fibres branch. The openings between the various sub- 

 divisions of the heart are guarded by valves which prevent a 

 return flow. 



In Scyllium the ventricle is produced forwards into a 

 muscular and contractile conus, which contains several rows 

 of valves. In front of this, the base of the ventral aorta is 

 swollen into a non-contractile bulbus. (The walls of the conus 

 contain heart-muscle, those of the bulbus smooth muscle.) 

 In the higher bony fish the conus tends to disappear while 

 the bulbus enlarges. Amia is primitive in showing a fairly 

 large conus with three rows of valves. In the Dipnoi, the 

 valves of the conus are well developed, and they give rise to 

 a spiral septum which almost or quite divides the conus into 

 two. These same forms are further very interesting in that 

 the ventral aorta is very much shortened up into a truncus 

 (instead of extending forwards all the way beneath the gills 

 as in Scyllium), and also because in Ceratodus there is a 

 beginning of the subdivision of the auricle into two, with the 

 pulmonary veins running into the left subdivision. 



In the frog, the heart is not unlike that of Ceratodus, except 

 that the auricles are completely divided into two, and that the 

 spiral septum in the conus and truncus is better developed, 

 dividing a pulmonary channel (leading to the pulmonary 

 arches) from an aortic channel (leading to the aortic and 

 carotid arches). 



In the water-breathing forms, the heart is always full of 

 venous deoxygenated blood, while in air-breathing forms there 

 is always a double stream of blood in the heart. One of these 

 streams is arterial and oxygenated, and the other venous and 

 deoxygenated. Since in the frog there is only one ventricle, 



