THE ARTERIES 609 



sive contact with the lateral wall of the thorax on the left side, ])ut none on the right 

 side, wdiere a con8i(leral)l(» tliickness of lung covers the pericardium.' 



Two bones, the ossa cordis, develop in the aortic fibrous ring. The right one 

 is in apposition with the atrio-ventricular rings and is irregularly triangular in 

 form. Its left face is concave and gives attachment to the right posterior cusp of 

 the aortic valve. The right surface is convex from before backward. The base 

 is superior. The posterior border bears two projections separated by a notch. It 

 is usually a little more than an inch (ca. 4 cm.) in length. The left bone is smaller 

 and is inconstant. Its concave right Ijorder gives attachment to the left posterior 

 cusp of the aortic valve. There is a large fleshy moderator band in the right 

 ventricle. 



The Arteries - 



The great arterial trunks in the thorax resemble those of the horse in general 

 disposition. 



The left coronary artery is much larger than the right one; it gives off a branch 

 which descends in the intermediate groove, and terminates by running downward 

 in the right longitudinal groove. The right artery, after emerging from the interval 

 between the right auricle and the pulmonary artery, divides into branches which are 

 distributed to the wall of the right ventricle. 



The brachiocephalic trunk (anterior aorta) is usually four or five inches (ca. 

 10 to 12 cm.) in length. 



The brachial arteries give off in the thorax the following branches: 



1. A connnon trunk for the subcostal, dorsal, superior or deep cervical, 

 and vertebral arteries. 



(1) The subcostal artery commonly arises separately, Ijut may be given off as 

 in the horse. It supplies the first three intercostal arteries. 



(2) The dorsal artery is relatively small. It usually ascends in front of the 

 first costo-vertel)ral joint and is distributed as in the horse. 



(3) The superior or deep cervical artery may arise from a common stem 

 (Truncus vertebro-cervicalis) with the vertebral, or may constitute a branch of that 

 artery. It passes up between the first thoracic and last cervical vertebra? or be- 

 tween the sixth and seventh cervical and is distributed as in the horse. 



(4) The vertebral artery passes along the neck as in the horse to the inter- 

 vertebral foramen between the second and third cervical vertebrae, gives off a 

 muscular branch, and enters the vertebral canal (Fig. 469). It runs forward on 

 the floor of the canal — connected with its fellow by two or three transverse anas- 

 tomoses — and divides in the atlas into two branches. The smaller internal division 

 (cerebrospinal artery) passes forward to the floor of the cranium and concurs with 

 the contlyloid artery and branches of the internal maxillary in the formation of a 

 large rete mirabile. The large external branch emerges through the intervertebral 

 foramen of the atlas and ramifies in the muscles of the neck in that region, com- 

 pensating for the smallness of the branches of the occipital artery. It also sends a 

 branch to the rete mirabile. The collateral branches detached to the cervical mus- 

 cles are large and compensate for the small size of the deep cervical artery. The 

 collateral spinal branches of the vertebrals pass through the intervertebral for- 

 amina, divide into anterior and posterior branches, and form two longitudinal 

 trunks which are connected by cross-branches so as to form irregular polygonal 

 figures. 



2. The internal thoracic artery. 



' It is, of course, only the pericardium which comes in contact with the wall, but it is cus- 

 tomary as a matter of convenience to speak of the relation of the heart as though it were direct. 



- Only the most important differential features of the arrangement of the vessels as compared 

 mth those of the horse will be considered. 

 39 



