The arch of the aorta (arcus aortas), commences at the upper part or base of the left ventricle of the heart, in front of the left auriculo-ventricular orifice ; it then passes upwards and to the right side, somewhat in the direction of the heart itself, crossing obliquely behind the sternum, and at the same time approaching more nearly to that bone; having gained the level of the upper border of the second costal cartilage of the right side, the vessel alters its course, and arches over from right to left, at the same time inclining backwards to reach the left side of the second dorsal vertebra ; here it makes another turn, and changes its direction so as to incline downwards upon the left side of the third dorsal vertebra, at the lower border of which the arch of the aorta becomes continuous (without any mark of separation) with the straight descending portion of the vessel known as the thoracic aorta.
In this course it will be observed that the artery describes a curve, having its convexity turned upwards, forwards, and to the right side, and its concavity of course in the opposite direction. A little above its origin the aorta is larger than elsewhere, and is not quite cylindrical, for it presents externally three small bulgings, of about equal size, which correspond with as many dilatations or pouches within ; these are named the sinuses of the aorta (arteriae magnse sinus, — Valsalva (Valsalvae Opera. Venetiis, 1740. Dissert. Anatom. i., sect, ii., p. 129, tab. 21)) ; they might be termed the sinuses of the aortic valves, in consequence of their connexion with the functions of those valves. In this place it may be mentioned, that in most cases there exists aling the right side of the ascending part of the arch a dilatation, named the great sinus of the aorta (arteriae magnae maximus sinus, — Valsalva). This partial dilatation of the vessel varies in size in different subjects, and occasionally is not to be detected.
Opposite to the three sinuses at the root of the aorta, of which two are anterior and one posterior, the section of the artery has a somewhat triangular figure; but immediately below them, where it is attached to the base of the left ventricle, the vessel is smaller, and its border circular. This circular border is connected with the margin of the aortic orifice of the left ventricle of the heart by means of a dense fibro-cartilaginous structure of an annular form, which enters into three indentations presented by the border of the vessel between the three sinuses. The aorta is also connected with the heart by the serous layer of the pericardium, which is prolonged for some distance upon the vessel, and by the lining membrane of the left ventricle (endocardium), which is continuous with that of the whole arterial system.
Around the inner side of the orifice of the aorta, and corresponding in position with the three sinuses, are attached three semilunar valves, the free margins of which meet together so as to close the mouth of the vessel, and prevent the reflux of the blood propelled from the ventricle into the aorta. Above two of the valves, and in the corresponding sinuses, are seen the orifices of the two coronary arteries of the heart, d e, the first branches given off by the aorta.
From the difference in the direction and connexions of different portions of the arch it is described as consisting of an ascending, a transverse, and a descending portion.
The ascending portion, of the arch of the aorta at its commencement is generally placed on a level with the upper border of the fourth costal cartilage of the left side, and it rises as high as the upper border of the second costal cartilage of the right side. Its length is about two inches or two inches and a quarter; and it is curved in its direction.
This portion of the aortic arch is enclosed in the pericardium, and, together with the pulmonary artery, is invested by a tube of the serous layer of that bag, in such a manner that both vessels are covered by the serous membrane, except where they are in contact one with the other.
At its commencement the ascending part of the arch is concealed by the pulmonary artery, and the right auricular appendage also over- laps it ; but as, in ascending, the aorta passes forwards and to the right side, whilst the pulmonary artery turns backwards and to the left, the former vessel comes into view, and approaches very near to the sternum, from which it is separated only by the pericardium and by cellular tissue and the remains of the thymus gland lodged in the anterior mediastinum. Higher up, the aorta has the descending vena cava on the right side, and the pulmonary artery (in passing backwards) on the left : behind, it is supported on the right branch of the pulmonary artery with the other constituents of the root of the right lung
The second, or transverse, part of the arch, is directed from right to left, and from the sternum to the body of the second dorsal vertebra. At its left side it is covered by the left pleura and lung, and is placed immediately in front of the trachea before its bifurcation into the bronchi. Here (on its posterior aspect) the aorta touches likewise the oesophagus, and is crossed by the thoracic duct. The upper border of this the transverse part of the arch has in contact with it the left innominate vein ; and from it are given off the large arteries (innominate, left carotid, and left subclavian), which are furnished to the head and the upper limbs. The lower border rests on the bifurcation of the pulmonary artery, and is connected with the left division of that artery by the remains of the ductus arteriosus. At or near its end this part of the arch is crossed in front by the left vagus nerve, and the recurrent laryngeal branch of that nerve turns upwards, beneath and behind it.
The descending portion of the arch, which is straight in its direction, rests on the left side of the body of the third dorsal vertebra, and is covered by the left pleura and lung. In front of this part of the arch is the root of the left lung ; and to its right side is the esophagus with the thoracic duct.
If the pulmonary artery be cut across at its root and drawn up- wards, it will be seen that the curve formed by the arch of the aorta encloses the pulmonary artery at its division, the root of the left lung, the left auricle of the heart, the left recurrent nerve, and the remains of the ductus arteriosus. — It may at the same time be observed that the aorta is connected with the pulmonary artery by the pericardium, and by the fibrous cord representing the ductus arteriosus of the foetus.
vol. I. 45
Peculiarities of the arch of the aorta
It is proposed to notice briefly under the head of peculiarities, all the more frequent deviations from the usual arrangement of each arterial trunk and its branches, and especially such as may be interesting in a surgical or physiological point of view. For more extended information on this part of the subject, reference may be made to a work on the arteries ( " The Anatomy of the Arteries of the Human Body, with its application to Pathology and Operative Surgery, by Richard Quain, in a series of plates and commentaries." London, 1844). In accordance with this plan, the peculiarities affecting the arch of the aorta will now be considered.
The height to which the arch of the aorta rises in the chest is liable to some variation. Although its highest part is usually placed about an inch below the upper margin of the sternum, it may reach very nearly to the level of that part of the bone ; and, on the contrary, it has been occasionally found an inch and a half below it, and (but this appears to be of very rare occurrence) as much as three inches from the same point. In these cases, the length and position of the great branches which spring from the arch undergo corresponding modifications.
Change of direction. — The aorta sometimes presents the singular peculiarity of arching over the root of the right lung instead of that of the left, and afterwards continuing on the right side of the vertebral column. In these cases, the viscera are all transposed, and the vena azygos is removed from the right to the left side. In other instances again, which are less frequently met with, this change in the direction of the aorta is only temporary, for after arching over the right bronchus (with the vena azygos), it resumes within the thorax its usual position, to the left of the vertebral column.
Peculiarities of Conformation. — The aorta has been observed to divide (Klinz in " Abhandlungen der Iosephinischen Med. Chir. Acad, zu Wien." Band ], S. 271, 1787 ; Troussieres in " Le Journal des Scavans," Paris, 1729), without forming any arch, into an ascending and a descending branch; the former of which ascended vertically for some distance, and then subdivided, like a cross, into three branches, to supply the head and upper limbs. This very rare disposition of the aorta corresponds with the usual arrangement in some quadrupeds (Hommel, in " Commercium Literarium," Hebdom. 21, Norimbergas, 1737; Mala- carne, " Delle osservazioni in chirurgia," &.C., part II. 1 19 ; Zagorsky, in " Mem. de l'Acad. Imp. des Sciences de St. Petersbourg," t. 9); and since, at an early period in the history of anatomy, dissections were for the most part prosecuted on the bodies of the lower animals, the terms ascending and descending aorta came also to be applied by the older anatomists to parts of the great systemic artery in the human subject.
Another very unusual change of conformation is that in which the aorta divides soon after its commencement into two large branches, which unite again into a single trunk, corresponding to the descending portion of the aorta (Usually the case in the pachydermata and ruminantia). In one case of this kind, (that recorded by Hommel), the trachea and esophagus were found to pass through the vascular ring formed by the divided aorta ; and they probably occupied the same position in the other cases also.
It is interesting to find that many of the peculiar conditions of the great systemic artery just mentioned, resemble the ordinary arrangements of that vessel in the lower vertebrate animals. Of this, one example in the case of quadrupeds, has been already mentioned. The change in the direction of the arch over the right instead of the left bronchus, is similar to the ordinary condition in birds ; and the bifurcation of the ascending aorta, and the subsequent reunion of its two parts has analogy with the arrangement of this vessel in the class of reptiles. It may further be remarked, that the mode in which these peculiarities may be supposed to be produced — by the pre-existence of certain fatal conditions of the vessel in the human subject, or by slight changes effected during the progress of development — has in many cases been satisfactorily shown.
The branches of the arch of the aorta taken collectively
The branches given off by the first part of the aorta are five in number. Two of these, named the coronary arteries, are comparatively small, and are distributed to the walls of the heart: they arise very near to the commencement of the aorta, and will be examined hereafter.
The other three branches from the arch are the three large primitive trunks which supply the head and neck, the upper limbs, and, in part, the thorax.
They usually arise from the middle or highest part of the arch, in the following order; first, the innominate or brachio-cephalic artery, which soon subdivides into the right subclavian and the right carotid arteries; secondly, the left carotid ; and thirdly, the left subclavian artery.
The origin of the left carotid artery is ordinarily somewhat nearer to the innominate artery than it is to the subclavian artery of its own side.
These three vessels, with the branches and the peculiarities presented by each, will be described in succession; but it is necessary so far to anticipate as to notice in this place the variations which have been observed in their mode of origin from the aorta, as connected with the peculiarities of that vessel.
Peculiarities of the branches arising from the arch of the aorta
Variations in the number and arrangement of the branches which arise from the aortic arch are of more frequent occurrence than those of the arch itself.
They may be arranged into two classes: 1. Those in which the primary trunks, viz., the carotid and subclavian, or the innominate arteries are concerned ; and 2. Those in which one or more secondary branches usually given from the subclavian, take origin directly from the aorta.
Peculiarities affecting the primary branches
These relate to the situation of the large branches upon the arch: to their contiguity to each other ¦ or to an alteration in their number or arrangement.
The situation of the branches
Instead of springing from the highest part of the arch, the branches are frequently moved altogether to the right, and take origin from the commencement of the transverse portion, or even from the end of the ascending portion of the arch. In these cases the vessels arise lower down than usual, especially the innominate artery; and they are generally crowded together on the aorta.
Their contiguity to each other
In the ordinary arrangement, the origin of the left carotid is nearer to the innominate than to the left subclavian; but the branches sometimes arise at equal distances from each other, or they are un-usually apart. A very frequent change consists in the approximation (in various degrees in different cases) of the left carotid towards the innominate artery.
The number and arrangement of the branches These are extremely various.
In a large series of observations the most frequent change met with in the number of the primary branches was their reduction to two. This most frequently arose from the left carotid being derived from the innominate artery (A natural condition in many apes and marsupialia).
In other cases of rare occurrence, the carotid and subclavian arteries of the left side (as well as those of the right) took origin by an innominate artery.*
On the other hand, the number of the primary branches has been found to be augmented to four, by the decomposition, as it were, of the innominate artery into the right carotid and subclavian arteries, which arose directly from the aorta.
In some of these cases the right subclavian artery, as might be expected, was the first branch given off from the arch, the right carotid, the left carotid and the left subclavian following in regular order; but this vessel was likewise found to take origin beyond one or two of the remaining three branches, or, as in most instances, beyond them all — from the left end of the arch; and other variations, in the order in which the branches arise, have been noticed.
Again, examples have occurred of augmentation in the number of the branches to five or six. In these cases the common carotid of one or both sides being absent, the external and internal carotid arteries arose from the aorta separately.
In other forms of variety, the number of the primary branches continues unchanged, but their arrangement is unusual. . Thus, when the aorta arches over to the right side, there may be three branches having the reverse of the ordinary arrangement, the innominate being on the left side. In other cases (the aorta having its usual course,) the two carotids have been seen to arise by a common trunk and the two subclavians separately — the right subclavian in most instances being transferred beyond the other branches to the left end of the arch. (Meckel mentions a rare case in which there were two innominatae, one of which sent off the two carotids, the other the two subclavians).
A very unusual change, referable to this form of peculiarity, observed by Tiedemann, consists in there being but one innominate artery, and that on the left side, although the aorta had its usual course over the left bronchus. — The subject was an infant, and had hare-lip.
Peculiarities in which one or more secondary branches, usually given from the sub-clavian, are derived directly from the aorta
In nearly all these, there is but a single secondary branch taking origin from the aorta; and such a branch, it may be remarked, has been found to accompany the ordinary arrangement of the primary branches, or to co-exist with a diminution or with an increase in their number ; that is to say, with all the conditions of the primary branches which have been just noticed.
The additional branch is almost invariably the left vertebral, which in nearly all such cases arises between the left carotid and left subclavian arteries; but it has been observed to proceed from the aorta beyond the last-named trunk.
A thyroid artery has been, though but rarely, seen to arise from the arch of the aorta.
Two secondary branches, taking origin from the aorta, have been very rarely met with. Those hitherto observed are the right internal mammary and the left vertebral ; or, as in one singular case, both vertebral arteries.
From Quain’s Anatomy