438 A HISTORY OF BIRDS 



regarded as so many more or less well-marked illustrations of 

 adaptation to function, the similar modifications of type re- 

 vealed by a study of the vascular system are more difficult of 

 interpretation, and with a brief reference to these this chapter 

 must close. 



The carotid arteries in birds, as in other vertebrates, are 

 typically two in number. But in many species one or other 

 of these arteries— right or left — atrophies, the remaining vessel 

 increasing in size in consequence. Yet again, as in certain 

 Parrots, the right carotid follows its normal course, along the 

 hypophyseal canal formed by the neck vertebrae, while the left 

 runs superficially along with the corresponding pneumogasiric 

 and jugular vein. In the Ground Hornbill both carotids have 

 this peculiarity, and are also of remarkably small calibre, their 

 work apparently being largely performed by the vertebral 

 arteries, which are unusually large. According to one author- 

 ity indeed the carotids are here reduced to the condition of 

 imperforate cords. 



Where only one carotid is present a vestige of the missing 

 artery is commonly found in the shape of a short canal leading 

 from the innominate artery to the persistent carotid. In the 

 Flamingo, for example, the left carotid only is present, the right 

 being represented only by a short tube leading from the right 

 innominate artery into the single carotid trunk. In the Cocka- 

 toos {Cacatua) the left is similarly curtailed. But in the Pas- 

 serine birds where the left carotid only is present the remnant 

 of the right has entirely disappeared. In the Bittern, on the 

 other hand, there is but a single carotid, but the bases of the 

 originally separate vessels are of equal size at their origin from 

 their respective innominate arteries. So far no explanation of 

 these vagaries appears to be forthcoming. 



The main artery of the thigh in birds shows similar varia- 

 tions. In most birds this is the femoral, but in some it is the 

 ischiadic. The Cuckoos of the Genus Centropus exhibit both 

 variations. Here, as in the case of the carotids, no interpreta- 

 tion has yet been found ; and the evolution of these peculiarities 

 seems indeed to have come about without regard to the struggle 

 for existence. 



