chap, v.] HEART OF VERTEBRATES. 195 



by similar structures ; these are numerous in the 

 lower, but reduced to a single set in the higher forms. 



The blood, on returning from the body to the heart, 

 is in the lower Vertebrata collected into an enlarge- 

 ment of the venous system which is known as the 

 sinus venosus, and the walls of this chamber are, 

 like those of the auricle and ventricle, contractile. 

 Contractility, then, is not, as in the higher members of 

 the group, confined to the centralised heart ; this is 

 well illustrated, on the one hand, by the Myxinoids, in 

 which the portal vein, and by the eel (McWilliam), 

 in which the terminations of the jugular veins, are 

 contractile ; and on the other, by the Elasmobrarich 

 fishes, the Dipnoi, and the Amphibia, in which the 

 basal portion of the aortic system (conus arteriosus) 

 is also contractile (Fig. 83). Here, as elsewhere, we 

 have evidence of gradation in the division of labour. 



The auricle, which is a single thin-walled sac in 

 most fishes, becomes more or less divided into two in 

 the Dipnoi; along the left-hand division of the heart 

 there flows, in addition to the blood from the veins, 

 that which has been returned from the rudimentary 

 lung (see page 232) ; along the right side the rest, or 

 purely venous, blood passes. Now, the arterial cone 

 or trunk arises rather from the left than from the right 

 side of the ventricle, which is incompletely divided 

 into two halves, so that the blood which first leaves it 

 is the blood from the left auricle ; this will, of course, 

 go to the farthest gill vessels, or those of the first and 

 second arch ; the last arch of all, the fourth, will, of 

 course, receive the most impure or venous blood, and 

 it is the one which, in Ceratodus, sends off a trunk 

 to the lung. 



This division of the auricle, which is hinted at 

 even in Chimsera (Lankester), becomes complete in 

 the forms which constantly breathe air by means of 

 distinct lungs, and the sinus venosus, which brings the 



