308 PRACTICAL ANATOMY. 



anterior cornu. Imbedded in it are a number of large 

 motor cells, which are intimately connected with the 

 anterior roots of spinal nerves. The posterior segment 

 of the gray matter, or posterior cornu, contains smaller 

 groups of polar cells, connected with the posterior roots 

 of spinal nerves. In the centre of the gray commissure 

 is the opening of the central canal, impervious in the 

 adult. It is lined by endothelium and opens just below 

 the calamus scriptorius of the fourth ventricle. In the 

 adult nothing remains of it except a pit called the fora- 

 men caecum, below the calamus scriptorius. The two 

 halves of the cord are also associated by the anterior and 

 posterior white commissures, which pass from side to side 

 in front and behind the gray commissure. The gray 

 matter of the cord is best developed in the cervical, next 

 in the lumbar, and least in the dorsal region. 



CRANIAL NERVES. 



Twelve pairs of nerves take their origin from the 

 brain. They are : 



1. Olfactory (special sense smell). 



2. Optic (special sense sight). 



3. Motor oculi (motion). 



4. Pathetic (motion). 



5. Trifacial (sensation and motion). 



6. Abducens (motion). 



7. Facial (motion). 



8. Auditory (special sense hearing). 



9. Glosso-pharyngeal (special sense and sensation). 



10. Pneumogastric (motion and sensation). 



11. Spinal accessory (motion and sensation). 



12. Hypoglossal (motion). 



1. OLFACTORY NERVE. Origin, by three roots: 

 (1) middle lobe of brain ; (2) from corpus striatum ; 

 (3) from gyrus fornicatus. These three roots unite, form- 

 ing the olfactory tract, which is about one-eighth inch 



