INTERNAL MAMMARY ARTERY. 265 



The chord of the curve of this artery, and the chord of the 

 curve of the aorta, are not in the same direction, but form an 

 angle with each other. - ^ 



The position of the Left Subclavian is somewhat different 

 from that of the right. Its origin is posterior, and, therefore, the 

 direction of the chord of its curve is more immediately lateral. 

 The curve or arch is also smaller. The situation of the two 

 subclavians as relative to the contiguous parts, is, therefore, 

 somewhat different ; but each of them proceeds between the 

 anterior and the middle scaleni muscles, and when they have 

 arrived at these muscles, their respective positions are very similar. 



The anterior and middle scaleni muscles arise from the trans- 

 verse processes of several of the cervical vertebrae, and are 

 inserted into the first rib, one before the other, so as to leave a 

 considerable space between them. The subclavian arteries 

 pass through this space, but before they arrive at it, and when 

 they are very near the above mentioned muscles, they send off 

 several very important branches in various directions, viz. to the 

 cavity of the cranium, to the parietes of the thorax, to the 

 thyroid gland, and to the lower part of the neck. 



They proceed near to the scaleni muscles before they send off 

 any branches ; and it is to be observed, that the subclavian veins 

 which correspond with these arteries, are anterior to them, for 

 they pass before the scaleni muscles, and not between them. 

 It is common, for greater clearness in description to divide the 

 course of the subclavian artery into three portions. 1st. That 

 between its place of origin and the .anterior scalenus muscle, 

 from which its main branches are sent off; 2. That immediately 

 behind the scalenus ; 3. That between the outer edge of the 

 scalenus and the lower border of the first rib, where the sub- 

 clavian takes the name of axillary. 



The Internal Mammary Artery 



Goes downwards, from the lower and anterior part of the 

 subclavian, along the inner side of the anterior scalenus muscle. 

 It proceeds, exterior to the pleura, across the cartilages of the 

 true ribs, and near the middle ; and, continuing between the 

 cartilages and the diaphragm, exterior to the peritoneum, 

 VOL. ii. 23 



