HEART AND ARTERIES OF VERTEBRATA. 585 



the subclavian, while on the left it gives rise to the aortic arch, 

 from which the left subclavian springs independently. The carotids 

 and the right subclavian are united to the commencement of the 

 aorta. In the Mammalia, therefore, the chief trunk of the arterial 

 system is formed by a left aortic arch. The pulmonary artery is 

 formed from the fifth arch, which is during foetal life connected 

 with the aortic arch of the left side by a ductus arteriosus 

 (ductus Botalli). The blood, therefore, of the superior vena cava, 

 which passes into the right ventricle, is in this way kept away from 

 the lungs, and is carried into the descending aorta, which till birth, 

 therefore, conveys mixed blood. After birth the communication 

 between the pulmonary artery and the aorta descendens disappears, 

 and the connecting portion {b) of this vessel is converted into a 

 chord (ligamentum Botalli). 



Sabatier, a., fitudea siir le coeur et la circulation centrale dans la serie des 

 vertebrea. Montpellier, 1873. 



§ 436. 



In all Vertebrata the systemic arteries arise, in their earliest 

 condition, from the bulbus arteriosus. In those forms that breathe 

 by gills the system of arterial arches, which arises from the bulb 

 (primitive aortic arches), is, as has already been remarked (§ 432), 

 broken up into the vessels of the branchial circulation; and the 

 system of systemic arteries is only derived from the efferent 

 branchial vessels (branchial veins). The current of blood, which at 

 first is sent to the aorta through the arterial arches, is driven into 

 new passages when the gills are developed, so that it is only dis- 

 tributed to the body after taking a roundabout course, which is due 

 to the conditions imposed by the respiratory process. 



In the Myxinoidea almost all the branchial veins unite to form a 

 subvertebral aorta, which is continued backwards as the chief artery 

 of the body, but which is also continued forwards as the " arteria 

 vertebralis impar." Two lateral longitudinal trunks are collected 

 from the branchial veins in just the same way, and each trunk sends 

 a branch anteriorly into the arteria vei'tebralis impar, while its other 

 branch goes to form a carotid. The two carotids divide into an 

 external and an internal branch, which supply the head. In Petro- 

 myzon the aorta is not continued forwards, so that the carotids, which 

 are formed in just the same way as in the Myxinoidea, form the sole 

 anterior arteries. In the Selachii and Chimaerte, the aorta is formed 

 from a trunk which is developed on either side by the union of the 

 branchial arteries. There is the same arrangement in the Ganoidei 

 and Teleostei. The carotids arise from the first branchial vein, or 

 from the anterior end of the paired arterial trunk, which collects the 

 branchial veins on either side to form the roots of the aorta, and 

 then unites with its fellow of the opposite side to form the aorta, or 

 enters anteriorly into a transverse anastomosis which mai-ks off an 



