CHAP, vii.] THE GAT'S ORGANS OF CIRCULATION. 209 



inner side of the anterior end of the petrous part of the temporal 

 bone, and unites with the circle of "Willis, formed by the branches of 

 the basilar artery. The internal carotid artery is so minute that it 

 would not be worthy of note were it not for its large size in man and 

 many other animals. 



§ i7. The SUBCLAVIAN ARTERY is the root artery of the fore-limb. 

 That of the right fore-limb springs from the larger branch of the 

 innominate artery. That of the left fore-limb takes origin directly 

 from the arch of the aorta. 



The subclavian artery first approaches the clavicle, and then, 

 arching over the first rib, recedes to the axilla and becomes the 

 axillary artery. 



The first branch given off by the subclavian is the vertebral arter//. 

 It bonds rather sharply upwards to pierce the transverse process of 

 the sixth cervical vertebra. It then advances through the perfora- 

 tions of the other cervical transverse processes, till, having traversed 

 that of the axis, it bends at a right angle to reach the notch between 

 the transverse process and the anterior articular process of the same 

 bone. It then traverses the atlas, and, finally, entering the cranium 

 through the foramen magnum, it unites (on the upper surface of the 

 basi-occipital) with its fellow of the opposite side to form the basilar 

 artery. 



The basilar artery runs forwards as a long single vessel, gives off 

 a branch on each side, and then continues on to form a circular 

 vessel around the infundibulum and pituitary body in its sella 

 turcica. This circular vessel is the circle of Willis, already men- 

 tioned ; the internal carotids unite with it, and branches to the 

 brain are given off anteriorly and laterally from it. 



The second branch given off from the subclavian on the same 

 side as the vertebral, is the thyroid axis. It arises covered in by 

 the scalenus muscle, and arches forwards and upwards over the 

 scapula — sending a branch up to the thyroid body. 



A considerable vessel, the internal mammary artery, is given off 

 directly opposite to the thyroid axis. It passes backwards along 

 the inside (dorsal surface) of the cartilages of the ribs. It gives 

 off a long and very slender branch, called the mperior phrenic, 

 which passes backwards to the diaphragm between the lung-root 

 and the pericardium. 



The superior intercostal artery, extends along the'inside of the first 

 three ribs, supplying the intercostal muscles. About half-an-inch 

 beyond the origin of the thyroid axis a second artery is given off, 

 which arches forwards and upwards over the scapula. 



§ 18. The subclavian becomes the axillary artery as it passes 

 into the axilla, between the pectoralis, subscapular, and latissimus 

 dorsi muscles, surrounded by the nerves of the brachial plexus. It 

 gives off various branches, as follows : the superior thoracic, which 

 goes to the pectoral muscle ; the acromial thoracic, which is large and 

 divides into advancing and receding branches which respectively supply 

 the deltoid, pectoralis and serratus magnus muscles ; the long thoracic 



