THORACIC AOETA. 401 



radial or the ulnar system of arteries, accompanied by a corresponding increase in the 

 other ; and it may be observed that the defect is much more commonly on the part 

 of the superficial, and the increase on the part of the deep set. 



(b) In a second and smaller class of variations a deficiency in one or other of the 

 two systems is supplied, either by the enlargement of branches which descend in 

 front of the limb, as the superficial volar (from the radial), or the median artery 

 (from the anterior interosseous), or by the enlargement of a metacarpal branch (from 

 the radial) on the back of the hand. 



In illustration of these general remarks, the following modes of arrangement of 

 the vessels may be mentioned. 



In the greater number of cases the superficial palmar arch is diminished, and 

 gives off fewer digital branches than usual. Generally only one branch is wanting, 

 viz., that which supplies the adjacent sides of the fore and middle fingers ; but some- 

 times two or three branches are absent, or even all four, as when the ulnar artery, 

 after giving branches to the short muscles of the little finger, ends in the deep pal- 

 mar arch. In the last-mentioned case, which is rare 3 it is obvious that the superficial 

 arch is altogether wanting. 



These various deficiencies in the superficial palmar arch and its branches are 

 usually compensated for by an enlargement of the deep arch, the palmar inter- 

 osseous branches of which, being increased in size, divide at the clefts of the fingers, 

 and form such collateral digital branches as are not derived from the usual source. 

 But a defective superficial arch may, as before mentioned, be reinforced from other 

 vessels, viz., from the superficial volar, from an enlarged median artery, or from a 

 large metacarpal branch. 



It sometimes, but more rarely, happens, that the radial system of vessels is defi- 

 cient ; in which case the superficial arch (which belongs to the ulnar system) may 

 supply all the digital arteries to the thumb and fingers, or one of these may be derived 

 from the superficial volar, the median, or the radial interosseous artery. 



DESCENDING AORTA. THORACIC AORTA. 



From the point at which its arch i3 considered to terminate the lower 

 margin of the third dorsal vertebra, the aorta descends along the fore part 

 of the spine to the fourth lumbar vertebra, where it divides into the common 

 iliac arteries. The direction of this part of the vessel is not vertical, for, as 

 it follows the bend of the spine, upon which it rests, it is necessarily con- 

 cave forwards in the dorsal region, and convex forwards in the lumbar. 

 Again, as its commencement is at the left side of the bodies of the vertebrae, 

 and its termination also inclined a little to the left, whilst about the last 

 dorsal vertebra the vessel is nearly upon the median line, there is produced 

 another slight curve, the convexity of which is to the right side. Within 

 the thorax, where the offsets are small, the aorta diminishes only slightly in 

 size ; in the abdomen the diminution is considerable, in consequence of large 

 branches being furnished to the viscera of that cavity. 



That part of the descending aorta which is situated in the thorax, is called 

 the thoracic aorta ; it extends from the lower border of the third dorsal 

 vertebra on the left side, to the opening between the crura of the diaphragm 

 in front of the last dorsal vertebra. It lies in the back part of the inter- 

 pleural space or mediastinum, being placed before the spine and behind the 

 root of the left lung and the pericardium ; on the left side it is in contact 

 with the corresponding pleura and lung, and close on the right side are the 

 azygos vein, the thoracic duct, and the oesophagus. The oesophagus, how- 

 ever, towards the lower part of the thorax is in front of the artery, and 

 near the diaphragm gets somewhat to the left side. The left or small azygos 

 vein crosses behind the thoracic aorta. 



The branches derived from the thoracic aorta are numerous, but small. 



