792 COMPARATIVE ANATOMY 



Circle of Willis: 



Just before the basilar artery reaches the hypophysis (pituitary 

 body) it divides, so that one-half of the basilar passes on each side of 

 the hypophysis. The internal carotid artery meets this divided basilar 

 on each side and the trunks thus formed meet near the optic chiasma 

 forming a complete arterial ring called the circle of Willis. It will be 

 noticed that the brain has thus a supply of arterial blood from both ven- 

 tral and dorsal regions, making it less likely to suffer from anything that 

 might impede the circulation in any one part. 



Subclavian Arteries: 



As the limbs grow out, a segmental artery for each somite concerned 

 in the appendages, extends into the member. These arteries become 

 connected with each other distally as well as with the veins of the limb 

 by a network of small vessels. The parts of these main trunks and the 

 connecting network enlarge while other portions atrophy. There are 

 numerous variations in the blood supply of the limbs. This explains 

 the shiftings of the subclavian artery shown in Figure 448. 



The subclavian has the following names applied to different por- 

 tions : 



Axillary that part lying in the axilla. 



Brachial that part lying in the upper arm. 



Radial and Ulnar the ones lying adjacent to the bones of these 

 names. 



Epigastric Arteries: 



The development of the arteries of the hind leg is somewhat com- 

 plex. There is the same formation of -a capillary network as with the 

 fore-limb. Two of the arteries become prominent. The epigastric 

 artery lies forward. It descends from the aorta to the ventral side of 

 the body and forward, supplying the lower portion of the myotomes. It 

 becomes connected with the epigastric veins at first, although later these 

 may anastomose with the hinder ends of the cutaneous arteries. When 

 the hind limb grows out, the external iliac or femoral artery (a branch 

 of the epigastric), is sent into its anterior side. As the leg increases in 

 size this sometimes surpasses the parent epigastric in size, so that the 

 latter appears as a mere side branch. 



Sciatic Arteries: 



The sciatic or ischiadic arteries descend into the posterior side of 

 the leg, the name changing at the angle of the knee to popliteal artery. 

 Farther down this artery divides into peroneal and anterior and posterior 

 tibial arteries. The peroneal supplies the calf of the leg and the others 

 continue into the foot. 



The arrangement of vessels here outlined is characteristic of the 



