COMPARATIVE ANATOMY OF THE CIRCULATORY SYSTEM 259 



near the mid-dorsal line of the thorax, giving off intercostal branches and then 

 supplying the deep muscles of the back. The other branch of the costocervical 

 axis leaves the thoracic cavity, passing deep dorsally in front of the first rib, 

 and divides into the transverse artery of the neck, supplying the serratus ventralis 

 and rhomboideus muscles, and the deep cervical artery to the epaxial muscles 

 of the neck. These branches are best found by looking among the muscles in 

 question and tracing the vessels toward the subclavian. The thyrocervical axis 

 generally arises anterior to the other branches. It passes forward near the 

 carotid artery and, after branching to the muscles of the dorsal side of the neck, 

 turns laterally in front of the shoulder, being then named the transverse scapular 

 artery; it accompanies the external jugular vein for a short distance and supplies 

 many muscles of the shoulder and neck. 



The subclavian artery now passes in front of the first rib into the axilla, 

 where it is named the axillary artery. This gives off: the ventral thoracic artery, 

 passing medially to the medial ends of the pectoral muscles; the long thoracic 

 artery, passing posteriorly along the middle region of the pectoral muscles and 

 then to the latissimus dorsi; and, near the arm, the large subscapular artery. 

 This gives off the thoracodorsal artery, lying parallel but more dorsal to the long 

 thoracic artery and supplying the latissimus dorsi; the subscapular then turns 

 dorsally, passes through the proximal part of the upper arm dorsal to the humerus, 

 and branches to the muscles of the upper arm and muscles of the back and 

 shoulder. 



The axillary artery then proceeds as the brachial to the medial surface 

 of the fore limb, where it accompanies the brachial vein and some nerves, and 

 branches into the limb. 



Draw the branches of the subclavian. 



b} Common carotid artery: The two common carotid arteries arise in the 

 cat from the brachiocephalic and immediately diverge; in the rabbit the right 

 one arises in common with the right subclavian, while the left usually springs 

 independently from the arch of the aorta. Trace the common carotids forward. 

 Their branches are similar in the two animals. They pass anteriorly in the 

 neck, one to each side of the trachea, to which they give small branches. At the 

 level of the anterior end of the thyroid gland each supplies a superior thyroid 

 artery to the gland. At the level of the larynx there are branches into the 

 larynx and adjacent parts (probably destroyed) and an occipital branch into the 

 dorsal muscles of the neck. The common carotid at about this same level gives 

 off the internal carotid artery. In the rabbit this artery arises at the place where 

 the carotid passes to the dorsal side of the shining ligament of the digastric. In 

 the cat it is much smaller and arises at the same level as the occipital artery. 

 In both animals the internal carotid passes dorsally in company with nerves and 

 enters the skull by a foramen through the tympanic bulla. It need not be 

 followed. The artery beyond this point is called the external carotid artery. 



