XII 



AYES 



433 



BirdSj along with Chelonians and' Crocodiles and a few mammals, 

 are characterized by the blood- 



TXC. 



'd.c. 



v.c. 



supply to the pectoral limb being 

 provided by a secondary subclavian 

 artery. The original or primary 

 subclavian artery is a branch of the 

 dorsal aorta or aortic root. In 

 the animals just mentioned the 

 blood-vessels in the limb rudiment 

 establish a secondary connexion 

 with the ventral end of the third 

 (carotid) aortic arch, and this 

 secondary channel usurps the place 

 of the original channel of supply 

 to the limb, forming the (secondary) 

 root portion of the subclavian 

 artery while the original root dis- 

 appears. 



In the venous system the main 

 point to notice is the disappearance 

 of the renal portal system which 

 was so conspicuous in the lower 

 vertebrates. Originally there is such 

 a system in the bird also, but during 

 the course of development the pro- 

 portion of blood reaching the tubules 

 by way of the renal portal becomes 

 less and less as compared with that 

 which comes direct from the dorsal 

 aorta. The advantage of this is 

 readily understandable when we 

 bear in mind the superiority of the 

 aortic blood both in composition 

 (arterial) and in pressure. 



Some of the peculiarities of the 

 bird's skeleton have already been 

 alluded to. The skeleton is very 

 highly ossified. The flexibility of the 

 neck is heightened by the peculiar 

 saddle-shaped joint surfaces of the centra. At the other end of the 

 vertebral column the tail region is reduced to a short stump, although 



2 F 



Fig. 185. 



Diagram illustrating tiie arterial system of 

 a bird as seen from the ventral side. Those 

 parts of the primitive arterial system which 

 are no longer present in the adult are drawii 

 in outline. A, Dorsal aorta ; an, anastomotic 

 vessel ; d.c, dorsal carotids ; I. A, left aortic 

 root ; l.p, left pulmonary ; p, pulmonary 

 r.A, right aortic root; r.p, right pulmonary, 

 S, systemic ; u.c. ventral carotid. 



