190 



AORTA. 



condary parts which lie in its vicinity, as the 

 thymus, the pericardium, the oesophagus, the 

 lenal capsules, ureters, &c. 4th order. Small 

 arterial twigs lost in the neighbouring cellular 

 substance, lymphatic glands, and in the coats 

 of the aorta itself. 



Development. --The aorta appears to be 

 formed in the foetus prior to the heart and sub- 

 sequently to the system of the vena porta, with 

 which, according to Baer, Rathke, and Meckel, 

 it is connected by a small dilatation described by 

 Dr. Allen Thomson* as a curved tube, which is 

 the rudiment of the heart. (See OVUM.) Whilst 

 the heart has but a single ventricle, the aorta and 

 the pulmonary artery form a common trunk, 

 which afterwards becomes divided by the de- 

 delopment of the contiguous portions of the 

 circumference of both vessels ; during the 

 remaining periods of intra-uterine life, and 

 for a short time after birth, the pulmonary 

 artery communicates with the aorta by the duc- 

 tus arteriosus, which appears as a continuation 

 of the trunk of the pulmonary artery opening 

 into the concavity of the arch of the aorta at its 

 termination. The ductus arteriosus becomes 

 impervious soon after birth, and having under- 

 gone a process of complete obliteration, is finally 

 concerted into a ligamentous cord. The size 

 of the arch of the aorta is less in proportion in 

 the foetus than in the adult, whilst the thoracic 

 aorta is larger, being increased in size below 

 the ductus arteriosus. The arch lies closer to 

 the spine in the foetus in consequence of the tra- 

 chea and bronchi behind it being so much less 

 developed than in the adult, and the thymus 

 which is between it and the sternum being 

 so much larger during foetal life. In old age 

 the curvature of the arch of the aorta is much 

 greater in consequence of the great sinus having 

 increased considerably in size. 



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. 

 Malacarnef 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 



* Vide Edinburgh New Philosophical Journal, 

 by Dr. Jameson, for October, 1830. 



t Osservazioni in Chirurgia. Torino, 1784. 



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.f 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.]; 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 eight. || 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 



* Comm. Noric. ann. 1737. 



t Vide Abhandlungen des Josephinischen Medi- 

 cinisch-Chirurgischen Akademie. Band, i. S. 271. 

 Taf. 6. Wien. 1787. 



t Meckel Handbuch der Menschlichen Anatomie. 

 Band iii. Halle and Berlin, 1817. Abernethy in 

 Phil. Trans. 1793. 



$ Desault in Journal de Chirurgie, torn. ii. 

 Dr. Goodison in Dublin Hosp. Reports. Brasdor 

 Recueil Periodique de la Societe de Medecine. 

 Paris, torn. iii. 



|| Handbuch der Menschlichen Anatomie. 

 Band iii. Halle and Berlin, 1817. 



