208 CIRCULATORY SYSTEM. 



aorta, they each go over the first rib of their respective sides, 

 adhering closely to it, in the bottom of the interval between the 

 scalenus anticus and medius muscles. The right subclavian is 

 much shorter, and more superficial than the left, from its origin 

 to the scaleni muscles. Near the latter they are each covered 

 in front by the sternal end of the clavicle, by the sterno-hyoid 

 and thyroid muscles, and fey the subclavian vein of the corre- 

 sponding side; behind they are separated from the vertebral co- 

 lumn by the longus colli muscle; below them is the pleura, the 

 left artery being in contact with it for its whole passage in the 

 thorax; and on their internal side is the primitive carotid. The 

 subclavian of the right side is crossed near the scalenus anticus 

 by the par vagum; the phrenic nerve also goes in front of it, 

 but on the internal edge of the scalenus. The subclavian of 

 the left side having a course almost vertical from its origin to 

 the interval of the scaleni muscles, is nearly parallel with and 

 behind, the primitive carotid of that side; the phrenic nerve has 

 the same relative position with it as on the right side ; but the 

 par vagum goes parallel with, and. in front of the subclavian 

 artery, for some distance along the root of the latter. 



At the inner margin of the Scaleni Muscles the Subclavian 

 gives off a cluster of trunks; to wit, the Vertebral ; the Inferior 

 Thyroidal ; the Superior Intercostal ; the Internal Mammary ; 

 and the Posterior Cervical Artery. They sometimes arise dis- 

 tinctly, and after the order mentioned : but there is too great a 

 diversity in subjects to establish any rule on these points. 



1. The Vertebral Artery (Arleria Vertebralis) is the most 

 voluminous of the branches of the Subclavian. Immediately af- 

 ter its origin it ascends on the side of the spine, and enters the 

 canal of the transverse processes of the neck at the sixth ver- 

 tebra. Pursuing this course, it gets into the cavity of the cra- 

 nium through the foramen magnum occipitis, and is distributed 

 to the brain in the manner mentioned in the description of that 

 organ. 



While in the canal of the transverse processes, it sends off 

 several branches to the heads of the contiguous muscles, and to 

 the medulla spinalis of the neck. The vertebral artery, like 

 some others, is spindle-shaped, its size augmenting as it recedes 



