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 vertebre 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 
Wilts 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 vertebre; the internal mammary 
artery, arising from the ventral side of the subclavian op- 
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