392 THE BRAIN. 



intervertebral foramina, except those of the sacral and 

 coccygeal nerves, which are found in the spinal canal. 

 Each of the spinal nerves, after leaving the intervertebral 

 foramen, divides into anterior and posterior branches. The 

 anterior, with the exception of the first two cervical, are 

 much larger than the posterior, and unite to form the sev- 

 eral plexuses, constituting the cervical, brachial, lumbar, 

 and sacral, which supply the muscles anterior to the spinal 

 column, and the upper and lower extremity. The posterior 

 supply the parts on the back of the spinal column. 



BLOOD-VESSELS OF THE SPINAL MARROW. (Fig. 8.) 



The spinal cord is supplied with arteries from the verte- 

 bral, intercostal, lumbar, and sacral. 



I'he vertebral, at the foramen magnum, sends off the 

 anterior and posterior spinal arteries, which descend the 

 whole length of the cord, in front and behind. Branches 

 of the intercostal, lumbar and sacral, enter through the 

 intervertebral foramina, to supply the cord. 



The veins are numerous. One on each side of the mid- 

 dle line, from its size, is called the sinus columnce ver- 

 tebralisy and is found on the posterior surface of the bodies 

 of the vertebrae, between them and the dura mater. 

 Branches running transversely join these sinuses ; and 

 there is also an anastomosis with the veins on the outside 

 of the spinal canal. 



SECTION n , 



THE BRAIN. (Fig. 16.) 



The brain is regarded as the great central portion of the 

 nervous system, and being situated within the cavity of the 

 cranium, is called the Encephalon, (wzsqahe, within the head.) 



The brain has four principal divisions, which we shall 

 examine in the natural order of their development, or from 

 below upward: 



1. Medulla oblongata. 3. Cerebellum. 



2. Pons Yarolii. 4. Cerebrum. 



