THE COROJs^AEY AETEKY. 353 



which have been observed ; the greater number of which it will be understood 

 from the nature of the change in the circulation which takes place at birth, are 

 only compatible with intra-uterine life. Such are those cases in which the 

 pulmonary arteiy leads through the ductus arteriosus, or fifth branchial arch of 

 the left side into the descending aorta ; while the aortic arch itself is completely 

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

 descending pai-t of the aorta. It is remarkable, however, that in some rare cases 

 of the kind now refeixed to, life has been prolonged after birth, and the ductus 

 artei-iosus 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 rise from 

 the arch and those connected with the descending aorta. 



Along with the same division may also be classed the series of converse cases, 

 in which the pulmonary arch being closed anteriorlj-, the pulmonary vessels have 

 received their supply of blood from the descending 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 in most rare cases of the right subclavian 

 artery from the ductus arteriosus or from one of the pulmonary aiieries, are 

 exi^licable by reference to the same group of developmental phenomena. 



5. The varieties in the number and jwsition of the rc%s('I.s- ajn-'niqinq from the 

 arch of the aorta are extremely numerous ; some very frequent, others compa- 

 ratively rare. These vessels may be all collected into one trunk, or they may 

 rise separately from the aorta to the number of six. In the rare case of one 

 trunk, we may suppose the anterior aortic roots to be combined so as to collect 

 all the branches proceeding from them together, as naturally occurs in what is 

 called the anterior aorta of the horse. 



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

 commonest of all the varieties of the aortic vessels, in which the left carotid is 

 united with the innominate artery into a common stem : and the other, com- 

 paratively rare, in which there are two innominate or brachio-cephalic tmnks, 

 as in birds. 



Three is the noi-mal number of branches rising separately from the arch in 

 man, and apes, and some other mammals. There is, however, a rarer 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 fonn of the condition in which there are four vessels rising 

 from the aortic arch, is that in which the sessile left vertebral artery nses 

 between the left carotid and subclavian arteries. A rarer form is that in wliich 

 the vessels rising 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 division of the innominate into 

 subclavian and carotid, together with separate origin of the left vertebral artery. 



In the case of six vessels proceeding from the single arch, of which there is an 

 instance described, the vessels were in the following order, which is tlaat which 

 might be anticipated from the mode of development, viz., right subclavian, 

 vertebral and carotid, left carotid, vertebral and subclavian ar-terics. 



There are, however, many other varieties and modifications of those already 

 mentioned, for which the fuller works already quoted may be referred to. 



BRft-NCHES OF THE AECH OF THE AORTA, 



The coronary arteries are two small vessels, named right and left, 

 which arise from the root of the aorta in the upper parts of the two 

 anterior of the three sinuses of Valsalva, on a level with the margins 

 of the semilunar valves. 



The right coronarij artery about the size of a crow's quill, runs 

 obliquely in the auriculo-ventricular groove towards the right side, and 

 thence along the posterior aspect of the heart, until it reaches the line of 



VOL. I, • A A 



