AORTA. 



191 



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 xipwards, 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 arterise innominate, 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 

 oft" 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; 6, 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 less frequent; the following are among 

 the more remarkable : o, the cceliac and dia- 

 phragmatic may arise above the diaphragm ; 

 one or both of the diaphragmatics may be 

 given off by the cceliac ; sometimes the cceliac 



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 

 occvirrence 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- 



* Dictionnaire de Medecine, art. Aortc. 



