VARIETIES OF THE AORTA. 



387 



vertebral is continued from the fourth arch, the connection of the latter with the dorsal aorta 

 being 1 obliterated (fig. 333, C). In such cases the inferior laryngeal nerve passes inwards to 

 the larynx around the first part of the vertebral artery. There are also a few examples of 

 the converse of this condition, the vertebral artery of the right side having a dorsal origin, 

 and passing behind the trachea and oesophagus to its usual place, while the subclavian is 

 continued from the fourth arch. 



Under the same division may be brought the numerous varieties in the closure of the 

 ductus arteriosus, and its union with the aorta or other vessels, which have been observed. 

 The greater number of these it will be understood, from the nature of the change in the 

 circulation which takes place at birth, are only compatible with intrauterine life. Such are 

 those cases in which the pulmonary artery leads through the ductus arteriesus, or fifth 

 arch of the left side, into the descending aorta, while the aortic arch itself is more or less 

 separated by a constriction or even a complete closure of its tube in the situation of the 

 aortic isthmus from the descending part of the aorta. It is remarkable, however, that 

 in some rare cases of the kind now referred to, life has been prolonged after birth, and 



Fig. 334. DIAGRAM OP THE NATURAL ORIGIN OP VESSELS FROM THE AORTIC 



ARCH AS COMPARED WITH THE DISPLACED SUBCLAVIAN ARTERY. (Allen 



Thomson. ) 



I, the normal disposition ; II, the right subclavian artery displaced or 

 proceeding from the dorsal aorta. A, A, ascending and descending parts of 

 the thoracic aorta ; P, pulmonary trunk ; d, ligamentum arteriosum ; a, part 

 of the right primitive dorsal aorta ; a', the corresponding part of the left side ; 

 c, common carotid arteries ; i. innominate artery ; s, right, and &', left sub- 

 clavian artery ; v, right, and v' left vertebral artery. j 



the ductus arteriosus having become closed, probably gradually, the 

 descending aorta has come to receive a full supply of blood from 

 enlarged anastomosing vessels (internal mammary, intercostal, &c.) 

 passing between the vessels which arise from the arch and those con- 

 nected with the descending aorta. 



Along with the same division may also be classed the series of 

 converse cases, in which the pulmonary trunk being closed below, the 

 right and left pulmonary arteries have received their supply of blood 

 from the aorta. Such examples of the origin of the pulmonary 

 arteries from the aorta, as they have been styled, and examples of 

 the origin of the left, or as a very rare occurrence of the right, sub- 

 clavian artery from the ductus arteriosus or from one of the pul- 

 monary arteries, are explicable by reference to the same group of 

 developmental phenomena. 



6. The varieties in the number and. position of the vessels springing 

 from the arch of the aorta are extremely numerous ; some very frequent, 



others comparatively rare. These vessels may be all collected into one trunk, or they may arise 

 separately from the aorta to the number of six. In the rare case of one trunk, we may 

 suppose the origins of the right and left arches to be united, and the fourth left arch to be 

 much shortened, thus bringing all the branches together, as naturally occurs in the horse, 

 forming what is called the anterior aorta. 



The cases of two vessels from the arch may be of two kinds ; in the one, which is the 

 commonest of all the varieties of the aortic vessels in man, and is the normal condition in 

 most apes, the left carotid is united with the innominate artery into a common stem ; in 

 the other, which is exceedingly rare, there are two innominate trunks, as in birds. 



Three is the normal number of branches arising separately from the arch in man and 

 some other mammals. There is, however, a rare form of variety in which the number is 

 the same, and in which, as occurs naturally in some cetacea, the subclavians are both separate 

 vessels, and the two carotids spring from a common stem in the interval between them. 



The commonest form of the condition in which there are four vessels taking origin from 

 the aortic arch is that in which the left vertebral artery arises between the left carotid and 

 subclavian arteries. The origin of four large arteries in the order, right carotid, left carotid, 

 left subclavian, and right subclavian, has been referred to (p. 386). A much rarer form is 

 that in which the vessels arising from the arch are successively the right subclavian, the 

 right carotid, the left carotid, and the left subclavian arteries. 



The number of five arteries proceeds from the separate origin of the right subclavian and 

 left vertebral arteries. 



In the case of six vessels proceeding from the single arch, of which there are a few 

 instances described, the vessels were in the following order, which is that which might be 

 anticipated from the mode of development, viz., right subclavian, vertebral and carotid, left 

 carotid, vertebral and subclavian arteries. 



n. 



