THE VASCULAR SYSTEM. 137 



The vertebral artery is the first branch given off by either 

 subclavian. It proceeds dorsad and craniad to the sixth- 

 cervical vertebra, whence it extends through the transverse 

 foramina of the cervical vertebrae and foramen magnum, to 

 a junction with its fellow in the median line on the ventral 

 aspect of the medulla oblongata. The union of the two 

 vertebral arteries forms the basilar artery, which, after giv- 

 ing off several branches to the medulla and cerebellum, 

 divides craniad of the pons Varolii, forming the circle of 

 Willis around the infundibulum and the optic chiasma at 

 the base of the brain. The circle of Willis receives the in- 

 ternal carotid and gives off several arteries to the cerebrum. 

 In man the internal carotid is much larger proportionately 

 than in the cat (Fig. 73). 



THE ARTERIES OF THE THORACIC LIMB. 



There is more or less variation in the branching of the 

 arteries in the limbs. No two cats are found exactly alike 

 as to their arteries or veins. The same is true of all other 

 mammals. 



The subclavian artery, which on the right side springs 

 from, or is merely a continuation of, the innominate and 

 on the left side arises from the arch of the aorta, sup- 

 plies the forelimbs with blood (Figs. 73 and 74). When 

 the subclavian reaches the armpit, it is called the axillary 

 artery, and its continuation along the humerus is the 

 brachial artery. A continuation of the same artery along 

 the radius is the radial artery, furnishing a large part of the 

 blood to the fingers. 



Four branches arise from the subclavian : the vertebral 

 artery, extending to the brain through the transverse for- 

 amina of the cervical vertebrae; the internal mammary 

 artery, arising from the ventral side of the subclavian op- 

 13 



