CIRCULATORY ORGANS. 



15 



bral foramina at the sixth vertebra. It passes through the transverse foramen In the 

 atlas, turns dorsally and runs along the groove between the transverse process and 

 the anterior expansion bearing the facet for articulation with the condyloid process, 

 and then goes through the foramen in the dorsal part of this to enter the cranial 

 cavity by the foramen magnum. After leaving the transverse foramen of the atlas it 

 gives off a branch backward to the muscles 

 of the back of the head and to the deep 

 muscles of the dorsolateral aspect of the 

 neck. The two arteries run forward along 

 the ventrolateral aspects of the medulla ob- 

 longata, but toward its anterior end they 

 converge and unite. The single median 

 trunk thus formed just back of the pons 

 Varolii gives off a branch on each side to 

 the cerebellum. The remainder goes for- 

 ward to the infundibulum, where it breaks 

 up into a number of branches, one on each 

 side to the anterior surface of the cerebellum 

 and the others to the tbalamencephalon and 

 prosencephalon. 



The thyroid axis is distributed to the 

 muscles of the neck and shoulder. Its first 

 branch is one given off outward, and curves 

 over the anterior aspect of the shoulder, and 

 then goes, posteriorly to the muscles on the 

 dorsal aspect of the same. In front of this 

 a second branch is given off inwardly. This 

 goes forward a short distance and then di- 

 vides into two vessels, both of which curve 

 over the anterior aspect of the shoulder to 

 the superior muscles of the same. The main 

 trunk turns dorsad and then backward, soon 

 to divide into numerous small vessels to the 

 deep muscles of the neck and shoulder. 



Each common carotid at the base of the 

 skull divides into an external and an internal 

 carotid, the latter entering the cranium by 

 the carotid canal. 



The abdominal aorta gives off a coeliac 

 axis, upper and lower mesenteric, renal and 

 genital vessels, and then divides into an ex- 

 ternal and internal iliac artery on each side. 

 In some cases the proximal ends of the in- 

 ternal iliacs form a short, common trunk. The external iliac gives off in the abdominal 

 cavity an epigastric artery, which runs forward on the ventral wall of the abdomen, 

 being distributed to the same, and then leaves the abdominal cavity and becomes the 

 femoral. The internal iliac runs backward laterally into the pelvic cavity. It gives 



■TflCi. 



Fra. 4.— Eight anterior venous system, 

 r. in. right innominate. 

 int.j. internal jugular. 

 ex.j. external jugular. 

 vert, vertebral vein. 

 az. azygos vein. 

 rfia. internal mammary vein. 



