REPTILES 201 



but the two kinds of blood do not mix here, for the party-wall 

 between the auricles projects into the opening, and divides it into 

 a right half and a left half. Although the cavity of the ventricle 

 is single, there is a well-marked ridge protruding into it which 

 must be regarded as an incipient partition, and actually divides 

 the ventricle when it contracts into a right half, mostly receiving 

 impure blood, and a left half, mainly containing pure blood. Im- 

 pure blood is pumped to the lungs through a pulmonary artery 

 arising from the right half of the ventricle, just as the same artery 

 is given off from the right ventricle in a Mammal or Bird (see 

 p. 40 and fig. 102). 



A notable peculiarity here deserves mention. In a Mammal 

 (see p. 41) the great body-artery, known as the aorta, starts 

 from the left ventricle in an aortic arch, which curves round to 

 the left, while in a Bird there is a similar arrangement, except that 

 the aortic arch curves round to the right. In the Lizard there is, 

 as it were, a combination of both conditions. There is a right 

 aortic arch, resembling a Bird's in that it arises from the left half 

 of the ventricle and curves over to the right. Since this arch 

 starts from that side of the ventricle which receives pure blood 

 from the left auricle, it has blood of that sort pumped into it. 

 And since the arteries supplying the head and fore-limbs are 

 branches of the right aortic arch, these parts of the body are 

 necessarily supplied with pure blood. And it is natural that the 

 brain above all organs should get a pure blood supply, which it 

 does in this way. 



But there is also a left aortic arch, curving as in the Mammal, 

 and conducting away some of the impure blood from the right half 

 of the ventricle. The dorsal aorta is formed by the union of right 

 and left arches, receiving pure blood from one, and impure blood 

 from the other, and therefore supplying what may be called mixed 

 blood to the trunk, tail, hind -limbs, and most of the internal 

 organs. Yet another point deserves attention. Each of the 

 aortic arches is double for part of its extent, and really represents 

 a pair of arches fused together. The presence of one or more 

 arches at the beginning of the great arteries of the body in ter- 

 restrial Vertebrates strikes one as peculiar, and without reference 

 to Fishes is inexplicable. As will later be explained, however, 

 it is an indication of descent from gill -bearing ancestors (see 

 p. 244), for the circulatory organs of lung-bearing forms are not 



