596 



EEPTILIA 



[CH. 



chiefly venous to the viscera, while the right-hand one supplies the 

 head, trunk and limbs with blood which is much more arterialised 



than that in the other arch 

 (p. 581). The fore-limbs 

 are however supplied by a 

 different vessel from the 

 subclavian of the Lizard. 

 We have already seen (p. 

 451) that in some verte- 

 brates the artery to the 

 fore-limb arises from the 

 systemic arch on its dorsal 

 course to join its fellow, 

 while in others the fore- 

 limb receives its blood from 

 an artery given off from the 

 ventral end or commence- 

 ment of the systemic arch 

 or else from the ventral 

 end of the third arch near 

 its division into dorsal and 

 ventral carotids. As the 

 vessel to the fore-limb is 

 always called a subclavian 

 artery it is convenient to 

 express the fact that this 

 vessel is not homologous 

 throughout the vertebrate 

 groups by the terms "dorsal 

 FIG. 296. Diagram of arterial arches of subclavian" and "ventral 

 a Turtle viewed from ventral surface. SUDC lavian." In Amphi- 



i, 11, m, iv, v, vi. First to sixth arterial -. i LiWrU th^ ^iih 



arches. 12. Tracheal (ventral carotid). Dian f an( ^ ^ lzam 

 13. Common carotid (dorsal carotid). 15. clavian is of the dorsal 

 Eight systemic arch. 16. Left systemic i . ov i 



arch. 17. Dorsal aorta. 19. Pulmonary. ^P 6 ' but m Chelonians 

 20. Innominate. 21. Subclavian (ventral and, as we shall see, in 

 type). 24. Coeliac. Crocodiles also, the arm is 



supplied by a ventral subclavian, a vessel which is homologous with 

 the "scapular" artery to the shoulder muscles in a Lizard. The 

 venous system in all chief respects is like that already described in 

 the Lizard. 



The copulatory organ is a grooved rod attached to the front 



