CIRCULATORY SYSTEM. 181 



rity of the intercostals) is too far removed from these anterior ribs 

 to send vessels to them ; their deficiency therefore becomes con- 

 veniently supplied by the anterior intercostal. These intercostal 

 branches follow the posterior margins of their correspondent ribs, 

 each running within a shallow groove; midway, however, between 

 the vertebra) and sternum, they leave the ribs for the middle 

 of the intercostal spaces, and end in several slender ramifica- 

 tions, some of which may be traced as low as the sternum, there 

 anastomosing with similar ones coming from the internal pectoral 

 artery. The intercostals in their course detach twigs to the 

 pleura, but more especially supply the intercostal muscles. 



2. The posterior cervical artery, arising in common 

 with the dorsal on the right side, but by a separate root on the 

 left, takes an opposite direction to that vessel, inclining forward 

 and upward. It traverses the upper part of the first intercostal 

 space, piercing the muscle filling it with considerable obliquity, 

 and then ascends in a winding course between the transverse pro- 

 cesses of the first dorsal and last cervical vertebrae, upon the body 

 of the latter, where it turns forward, and runs as high as the 

 vertebra dentata, close alongside of the roots of the ligamentum 

 nuchae, concealed by the complexus major, to which its ramifica- 

 tions are principally distributed. Besides some small and unim- 

 portant branches within the chest, it gives off the Just intercostal 

 arteri/, which descends behind the first rib, and anastomoses with 

 twigs from the internal pectoral. 



3. The vertebral artery is a vessel of considerable size, 

 and is vastly important, from being one of the principal conduits 

 of blood to the brain. It arises with a sort of bend from the 

 upper part of the arteria innominata, directly opposite to the first 

 rib, in such manner, that, if a knife were thrust into the chest behind 

 the rib, the artery would just escape puncture. The right vertebral 

 is at its origin placed lower than the left, and deviates somewhat 

 less from a straight line in proceeding, which it does, close under 

 the transverse process of the seventh cervical vertebra to enter the 

 foramen through that of the sixth. They then both of them con- 

 tinue their passage to the head, passing directly through the fo- 

 ramina of the fourth, third, and second vertebrae, wherein they 

 run securely barred from external injury : arriving at the atlas, 

 they make a curve upward upon its transverse processes, proceed 

 through the posterior pair of foramina, and enter the foramen 

 magnum. In its course along the neck, the vertebral artery on 

 either side detaches several short branches of considerable size, 

 which turn round the transverse processes, and ramify among the 

 deep-seated muscles. It also sends some smaller ones inward, 

 which enter the intervertebral holes fur the supply of the medulla 



