THE NERVOUS SYSTEM 203 



lateral columns are divided in their function between mo- 

 tion and sensation. 



The spinal cord terminates, at its lower extremity, in 

 an oval tubercle, whence arise a number of nerves that 

 go tp the lower parts of the body. From the appearance 

 these numerous nerves assume as they emerge from the 

 spinal cord, they have collectively been denominated, the 

 "cauda equina," from a fancied resemblance to a horse's 

 tail. The spinal cord, as well as the medulla oblongata, 

 is enveloped in its entire length by the continuation of the 

 three membranes of the brain. A more detailed descrip- 

 tion of the anatomic structure of the various portions of 

 the nervous system will be given further on. 



THE NERVES 



The nerves are tubular cords, the substance of whose 

 structure is similar to that of the brain and spinal cord. 

 All nerves take their origin in the brain and the spinal 

 cord, and extend in sets to every part of the body by differ- 

 ent routes. They often unite in their course and form a 

 plexus. There are 43 pairs of primary nerves arising 

 from the nervous centers; these, according to their origin, 

 are termed cranial, or encephalic, and spinal nerves. 

 Twelve pairs originate within the skull and 31 pairs 

 pass from the spinal cord 8 cervical, 12 dorsal, 5 lum- 

 bar, 5 sacral, and i coccygeal. 



Each nerve is composed of several filaments or cords, ly- 

 ing alongside one another, and surrounded by the neuri- 

 lemma. Most of the cranial nerves have but a single root, 

 but the spinal nerves arise by two roots one from an 

 anterior fasciculus of filaments and the other from a pos- 

 terior; these are separated from each other by the liga- 

 mentum denticulatum. The two roots later join, however, 

 to form one nerve. 



The spinal nerves, as has just been stated, arise by 



