■3j0 the aorta. 



aorta below the superior mesenteric artery (cut sliort) and the origin of the renal arteries ; 

 JO, little below this the origin of the spermatic arteries ; below //, the inferior mesenteric 

 artery, 11, 11, two of the lumbar arteries. 



heart, comparative!}' small, arise from the two anterior simiscs of Valsalva, 

 .unci are distributed to the walls of the heart. The other three are large 

 primitive trunks, which supply the head and neck, the upper limbs, 

 and, in part, the thorax, and usually arise from the middle or highest 

 part of the arch, in the following order : — first, the innominate or 

 hrachio-cophaJlc artery ; second, the left carotid ; and, third, the Jeff 

 .subclavian artery. The origin of the left carotid artery is usually 

 somewhat nearer to the innominate artery than it is to the subclavian 

 .artery of its own side. 



Varieties. — It will be proper in this place to refer to the varieties -uiiicli affect 

 the whole aorta, as well as to those of the pulmonary arteries. The former may 

 lie distinguished, according as they occur in the whole length of the vessel, or 

 lielong to one or other of its parts, those which are very frequent in the arch 

 Toeing especially deserving of notice. These last are of peculiar interest, but as 

 the full explanation of their mode of formation is connected with the historj-^ of 

 foetal development, the reader is referred to the chapter on that subject for 

 elucidation of the present outline of the nature of the varieties which is intro- 

 <Iuced in this place. 



1. The Aorta may vary in its position and extent. Thus the height to which 

 the arch rises in the upper part of the chest is found to be subject to variation to 

 the extent frequently of from one to two vertebral spaces ; more rarely to a 

 greater extent, so that while in some instances the summit of the arch has been 

 on a level with the top of the sternum, in other cases it has been as low as the 

 fourth or fifth dorsal vertebra. 



The distance to which the aorta extends downwards depends on the seat of its 

 division into the common iliac trunks, which frequently varies to the extent of 

 one of the lumbar vertebras, so that the place of division may he as low down as 

 on the fifth, or as high up as on the third. In other rarer cases the division 

 occiu's still higher. 



The position of the aorta with reference to the middle line or vertebral column 

 is also subject to some variation, liut such deviation to the side is more frequently 

 ihe result of pathological changes than of congenital malformation. 



A very remarkable malformation of the aorta consists in the greater or less 

 division of the vessel through a part or the whole of its channel into two 

 closely united tubes, by a median septum running through the cylindrical tube 

 from before backwards, or slanting from side to side, as m the cases observed by 

 .Craveilhier, Vrolik, Schroder Van der Kolk, and Allen Thomson, which when not 

 due to pathological changes may admit of explanation on the supposition of the 

 fusion of the origmal double embryonic aorta having remained incomplete. 



2. The Vtir'u'tir.s of the t<trmx, or main trunks of the aorta and pulmonary 

 ■artery, are intimately connected and usually associated with malformations of the 

 heart, and frequently with the persistence of the ductus arteriosus. These first 

 parts of the two great arteries, specially enclosed by the pericardium, are derived 

 from the arterial bulb of the foetal heart, and are liable to variations which may 

 be traced to deviations from the natural mode of their septal division, and of 

 their union with the left or right ventricles of the heart respectively. Thus these 

 two arterial trunks may be transposed, or each one may be connected with the 

 ventricle to which it does not naturally belong. I.e., the pulmonary artery with 

 the left, and the aorta with the right ventricle. Or the arterial tiimks may 

 communicate together more or less freely by deficiency of the septum between 

 them. Or one of the vessels may be nearly or entirely obliterated : while the 

 other, from unnatural openings left between them, serves as the channel for the 

 .stream of blood belonging to both vessels. Or the aorta and pulmonary arteries 

 may be entirely united in one simple stem in connection with a simple heart 

 .■similar to that of fishes. 



