148 ELEMENTS OF MAMMALIAN ANATOMY. 



the vertebrarterial canal 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 

 giving off several branches to the medulla and cerebel- 

 lum, divides cephalad of the pons Varolii, forming the 

 circle of Willis around the infundibulum and the optic 

 chiasma. The circle of Willis receives the internal ca- 

 rotid and gives off several arteries to the cerebrum. In 

 man the internal carotid is much larger proportionately 

 than in the cat (Fig. 71). 



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 brachiocephalic, 

 and on the left side arises from the arch of the aorta, 

 supplies the forelimbs with blood (Figs. 71 and 72). 

 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 vertebrarterial 

 canal; the internal mammary artery, arising from the 

 ventral side of the subclavian opposite the origin of the 

 vertebral, and extending along the visceral surface of 

 the sternum, and sometimes supplying the pericardium 

 of the heart; the superior intercostal, arising near the 



