Anomalies.—The aorta presents occasional varieties or anomalies in the mode of its origin, its course, termination, and the number and situation of its branches. It is an interesting fact, that almost every irregularity hitherto observed in the course and branching of the aorta in the human subject, represents the dis position which that vessel constantly exhibits in some of the inferior animals. The anomaly of the aorta arising from both ventricles, and causing that condition called cyanosis, will be more properly considered in the article HEART. The following anomalies of the course of the aorta have been recorded by anatomists : 1st. The aorta sometimes divides imme diately after its origin into a right and left trunk, which, after having each given off the arteries of one side of the head and one upper extremity, join to form the descending aorta. Malacarnet has described a remarkable case of this anomaly ; the aorta was of an oval form at its origin, its greater diameter being to its lesser in the proportion of three to two, it had five sigmoid valves in its interior, it divided immediately after its origin into a right and left trunk, from each of which arose a subclavian, an external and an internal carotid : after the two trunks had run for a space of four inches distinct, they joined to form the descending aorta. Hommel, a Norwegian ana tomist,* relates a case in which the transverse portion of the arch of the aorta divided into two trunks, one of which passed before and the other behind the trachea, after which they joined to form the descending aorta, having encircled the trachea with a sort of ring : this anomalous division of the arch of the aorta is the more remarkable as it approaches the con dition of the vessel which is constant in all known reptiles. 2d. The arch of the aorta is sometimes absent, in consequence of the vessel dividing, immediately after its origin, into two great trunks, one of which gives off the arte ries of the head and upper extremities, whilst the other becomes the descending aorta.t This distribution is similar to that in the horse, rhinoceros, and other pachydermata, in the ruminantia, and some of the rodentia. 3rd. Varieties in the course of the arch sometimes, although rarely, occur, as, for instance, when the arch of the aorta, instead of crossing to the left in the usual manner, curves over the right bronchus, and gets to the right side of the spine, whence it either immediately crosses behind the trachea and oesophagus to the left, or continues its course along the right side of the spine to the lower part of the thorax ; in cases of complete transposition of the vis cera, where the heart is in the right side of the chest, the arch of the aorta is also reversed, in which case its thoracic portion descends along the right side of the spine.X Instances are recorded in which the descending aorta, a little below its arch, was very much con tracted in its area or even completely obliterated for a certain distance, below which it resumed its full size : the circulation in these cases was carried on by the anastomosing of large col lateral branches arising above and below the constricted or obliterated part.§ Anomalies of the branches of the aorta are more frequent : according to Meckel the branches arising from the arch deviate from the normal condition in one person out of every eightli The branches arising from the arch of the aorta present three kinds of ano maly, which, as to their frequency, occur in the following order : 1st, an increase in their num ber; 2d, a diminution; and 3d, an anomaly in the identity or order of the branches arising from this part without any increase or diminu tion of their number. In anomalies of the first kind, the number of branches is most fre quently increased to four, by the left vertebral arising from the arch between the left carotid and left subclavian, as in the phoca vitulina ; next to this in frequency is the instance of the inferior thyroid arising from the arch between the innominata and left carotid, then the in ternal mammary, and, lastly, the most un usual is the thymic artery : it is more unusual to find the number of branches coming from the arch increased to four, in consequence of the innominata being absent, the right carotid and right subclavian arising separately ; in such a distribution the right subclavian most fre quently arises from the left extremity of the arch after the left subclavian ; it may, how ever, be the first branch of the arch to the right, or it may arise between the two carotids, or, as more rarely happens, between the left carotid and left subclavian. The number of branches arising from the arch will be in creased to five or upwards, when two or more of the above-mentioned anomalous branches arise from it at the.same time. Of the second kind of anomaly, or that by diminution of the number of branches, the most frequent is where these are reduced to two, of which there occur the following varieties : a. the in nominata sometimes gives off the left carotid as an additional branch, and the left subcla vian arises separately, as in many quadrumana, several of the carnivora, as the lion, cat, dog, weazel, several rodentia, &c.; b. sometimes there are two arterize innominatze, each dividing in a symmetrical manner into the subclavian and carotid of its own side, as in cheiroptera and the dolphin ; c. sometimes when the arch gives off but two trunks, one of them divides into the two carotids, and the other into the sub clavians ; d. the right subclavian may arise
distinct, and a common trunk give off the two carotids and left subclavian.; the origin of a single trunk from the arch of the aorta sup plying the arteries of the head and upper extremities is equivalent to a division of the aorta into an ascending and descending trunk, already noticed. The third kind of anomaly partakes of the characters of the two pre ceding, although the number of branches is the same as in the normal state : its varieties are, a, the left vertebral arising from the arch, whilst the left carotid comes from the inno minata; b, the two carotids may arise from a common trunk between the origins of the right and left subclavians, as in the elephant ; c, the right subclavian and right carotid may arise as distinct branches, whilst the left carotid and left subclavian come from a common trunk, forming a complete inversion of the usual order ; d, the left carotid may arise from the innominata, whilst the right carotid comes from the part of the arch in the situation usu ally occupied by the origin of the left carotid.
Anomalies of the branches of the descending aorta are lessirequent; the following are among the more remarkable : a, the coeliac and dia phragmatic may arise above the diaphragm one or both of the diaphragmatics may be given off by the coeliac ; sometimes the coeliac and superior mesenteric arise by a common trunk as in the tortoise; sometimes there are two or more renal arteries on one or both sides, and sometimes the primitive iliacs are given off much higher than usual, in which case they are sometimes connected by a cross branch before they divide into the external and in ternal iliacs : it sometimes happens, when the iliacs are given off higher than usual, that the inferior mesenteric arises from the left of them.
The diseased conditions of the aorta are described in the articles ARTERY and HEART. The aorta, as Beclard remarks,* is more sub ject than any other artery to the ovoid dila tation in its ascending, and the lateral dila tation in its descending portion ; it is also very subject to osseous or calcareous deposits, to fissures and ulcerations, to tubercles and small abscesses in its parietes, and to aneurism. Wounds of the aorta are constantly mortal. Laennec has observed a particular lesion of this vessel ; it was a fissure of the internal and middle coats, from which the external tunic was extensively separated by a quantity of blood which had been effused between it and the middle tunic. The late Mr. Shekelton has described, in the Dublin Hospital Reports, a form of aneurism of the lower part of the abdominal aorta, in which the blood forced its way through the internal and middle coats, dissected the middle from the external for the space of four inches, and then burst into a lower part of the canal of the artery, forming a new channel which eventually superseded the old one, which the pressure of the tumour obliterated.
Granular excrescences are sometimes formed on the valves of the aorta, which Corvisart conjectured to be of venereal origin. The in ternal tunic of the aorta sometimes presents a red appearance, not peculiar, however, to this vessel, and occurring in certain forms of fever. Obliteration or constriction of the aorta. is a condition rarely met with ; its existence may be traced either to pressure on the vessel from without, morbid thickening of its coats, or the formation of coagula internally ; this latter occurrence being most usually a consequence of the spontaneous cure of aneurism.
Aneurisms of the aorta produce various effects on surrounding parts ; thus the heart, lungs, trachea, oesophagus, pulmonary artery, large veins, thoracic duct, and the various organs in the abdomen placed in their vicinity, may suffer derangement of their functions, displacement, atrophy or partial destruction, according to the degree of pressure to which they are subjected.
Aneurisms occurring in the ascending por tion of the aorta, which is within the pericar dium, are often attended during life by many symptoms very similar to those of disease of the heart itself, while their pressure may produce a diminution of the calibre of the pulmonary artery, obstruct the free passage of the blood through the vena cava superior, and even in , terfere with the full distension of the auricles. Aneurisms of the transverse portion of the aorta, when directed forwards, usually project at the right side of the sternum about the second intercostal space : when the sac extends upwards towards the neck, it frequently be comes a matter of extreme difficulty to dis tinguish an aneurism of the aorta from an aneurism of the innominata or some other large arterial trunk in the neighbourhood ; cases are on record, where the pressure of such aneurisms of the aorta caused obliteration of the subclavian and common carotid. When aneurisms extend backwards, they produce a variety of effects, interfering with respiration and deglutition from their pressure on the trachea and oesophagus, sometimes producing obliteration of the thoracic duct. The pres sure produced by aneurisms of the thoracic and abdominal aorta occasionally cause ab sorption of the bodies of the vertebra, and give rise to an appearance not very dissimilar to that produced by caries.