THE THYROID AXIS. 



237 



median descending artery, which is continued by anas- 

 tomosis the entire length of the spinal cord. 



Posterior Spinal. Unites with its fellow and forms 

 through anastomosis the descending median spinal artery. 



FIG. 106. BASE OF BRAIN, SHOWING CIRCLE OF WILLIS. 



I, olfactory bnlb ; 2, optic nerves ; 3, anterior perforated space ; 4, tractns opticus ; 5, 

 cms cerebri : 6. third pair of nerves : 7, fourth pair of nerves ; 8, fifth pair of nerves ; 9, 

 sixth pair of nerves: 10, pyramid: 11, olivary body: 12. vertebral artery: 13, anterior 

 spinal artery ; 14, anterior cerebral artery ; 15. lamina cinerea : 16. middle cerebral artery ; 

 17. tuber ci'nerenin: 18, corpora albicantia: 19, posterior perforated space; 20. posterior 

 cerebral artery : 21, anterior eerehellar artery : 22. pens Varolii; 23. posterior inferior artery ; 

 24, seventh and eighth pairs of nerves; 25, ninth, tenth, and eleventh pairs of nerves; 26, 

 twelfth pair of nerves ; 27, cerebellum. 



Posterior Meningedl. Enters foramen magnum, and 

 is distributed to the meninges. 



Posterior Inferior. Winds around the upper part 

 of medulla and is distributed to the cerebellum. 



