HEART AND ARTERIES OF VERTEBRATA. 



587 



given off on either side of a common trunk (G) . They generally appear 

 as two trunks, which pass along the sides of the neck in company 

 with the vagus. In the Saurii they do not lose their connection 

 with the next arterial arch, and retain, therefore, their primitive 

 relations. In many Ophidii the right common carotid is atrophied, 

 and may even completely disappear. 



Fig. 332. Development of the great arterial vascular trunks, as seen in the embryoes, 

 A Of a Eeptile (Lizard), B Of a Bird (Fowl), and C Of a Mammal (Pig). The 

 two first pairs of arterial arches have disappeared in all three ; in A and B the third, 

 fourth, and fifth are still persistent; in C the two last are alone complete, and the 

 third is no longer connected with the fourth pair. A branch (p) of the fifth forms the 

 pulmonary artery. Its trunk from this point to the aorta forms the ductus Botalli. 

 c Carotis externa, c' Carotis interna. In A and B this forms the anterior prolongation 

 of the root of the aorta, and in C a common trunk with the external carotid. 

 a Auricle, v Ventricle, ad Aorta descendens. s Branchial clefts, m Rudiment of 

 the fore-limbs, n Nasal pit (after H. Bathke). 



In Birds also this artery arises in company with a subclavian 

 from a common trunk (art. brachiocephalica), hut it leaves its 

 primitive course, and lies in the middle line of the inferior surface 

 of the cervical vertebras ; on the left, however, it retains its original 

 course. In others, again, the two carotids both leave their old 

 course, and this leads on towards that third form, in which the 

 two closely-approximated vessels are fused together. In this case 

 no part of the right carotid runs by itself, but a vascular trunk 

 arises on the left side, which runs in the middle line, and which 

 passes to the head, under the name of the primary carotid. Many 

 Birds have this character in common with the Crocodilini. A single 

 carotid trunk (Fig. 329, c), which obtains in the Ophidii and various 

 'Saurii, must be regarded as differing from this, although it also 

 passes into two cephalic arteries anteriorly. This arrangement is 

 due to the approximation of the points of origin of the two carotids 

 on the right aortic arch. A common arterial trunk is given off from 

 the united point of origin. Another peculiarity is the presence of 

 an unpaired subvertebral artery, which runs forward from the right 

 aortic arch along the vertebral column (Fig. 329, sv). 



