VAEIETIES OF THE AORTIC ARCH. 



351 



B. The Viu-k'tifs Ui the Aortic Arch itself, along with which must be included 

 those of the ductus arteriosus, are intimately connected ■ndth the mode of develop- 

 ment of the fourth and fifth foetal branchial arteries. The natural aortic arch of 

 man. and of all mammalia, is a left one produced by the persistence and develop- 

 ment of the fourth left branchial arch : in birds it is the right arch which 

 forms the permanent aorta ; and in reptiles both the right and left fourth arches 

 remain patent. 



Here it maj' be proper to call attention to the complete lateral transpositioiu i. e., 

 from right to left and vice verm, which affects the aortic arch and pulmonary- 

 vessels, as well as the other parts of the heart, when transposed with or without 

 the transposition of other viscera. Several cases of this kind have been accurately 

 recorded by various observei-s, and are usually unattended with any disturbance 

 of the functions or otherwise unnatural condition of structure. There is, in fact, 

 only a change of position, which may be best described by comparing it to 

 that in which the natural parts would appear if viewed by reflection from a 

 mirror. Although this transposition gives rise to no perceptible lesion of function, 

 yet from the direction of the apex of the heart towards the right, and other 

 aifferences from the natural position, its existence is capable of being ascertained 

 duiiag life. 



Fig. 244. — Diagram op the Fcetal Aortic Fig. 244. 



Arches, showing their transforma- 

 tions INTO THE PERMANENT VESSELS OF 



THE Mammal (after Ratlike). A.T. 



A, to the riglit of the primitive aiierial 

 bulb, now divided into aortic and i)uImo- 

 nary stems, the latter in front ; «, the right, 

 ■a', the left aortic roots ; A', the descending 

 aorta ; on the riglit side, the double out- 

 lines, 1, 2, 3, 4, 5, indicate the five primi- 

 tive branchial vascular arches ; on tlie left 

 side, I, II, III, IV, mark the seat of tlie 

 four branchial or pharyngeal clefts ; c, the 

 place of division of the aortic arches into 

 two anteriorly, between the common carotid 

 arteries ; cc, the permanent external caro- 

 tids ; ci, the right, ct', the left internal 

 carotids ; s, the right, s', tlie left subcla- 

 vians ; r, the right, v' , the left vcrtebrals ; 

 the fourth right arch forms the innominate 

 trunk, and passes on to v and s, the right 

 vertebral and subclavian arteries ; the fourth 

 left arch passes to a' as permanent aortic 

 arch ; P, pulmonary arteries springing from 

 the fifth left arch, which at d is continued 

 into the left aortic root as ductus arteriosus ; 



pn, right, 'pti' , left pneumogastric nerves. Tlie permanent systemic arteries are repre- 

 sented in deep shade. 



The aortic arch has been observed complcichj douUe in two remarkable cases, 

 known as those of Malacarne and Hommel. and various less complete cases of a 

 similar kind have been observed. Hommel's case admits of being explained on 

 the supposition simply of both the right and left foui-th branchial arches having 

 remained pervious, and undergone equal development. The pulmonary artery was 

 situated to the left of the main aortic stem : the right and left aortic arches 

 embraced closely the trachea and gullet m a ring, and each arch gave rise to a 

 common carotid and subclavian artery in the order now mentioned. Malacarne" s 

 case is different, and seems to have been complicated with some remarkable 

 abnormal mode of development of the arterial stem. The two arches, as in 

 Hommel's case, embraced the trachea and gullet, but they divided close to the 

 heart, so as to receive the pulmonary stem between them : and each gave rise to 

 a subclavian, an external, and an internal carotid artery, in the order now stated, 



