298 



THF INTFRNAL ENVIRONMENT OF THE BODY 



Part III 



Central Nervous System 



The spinal cord is the main connection between the brain and all parts of 

 the body except regions of the head. It varies greatly in length, extends to 

 the end of the body in fishes and snakes, is shortened in mammals, and 

 reaches only to the small of the back in the human body. It contains a 

 central fluid-filled canal, a remnant of the once open gutter of the embryonic 

 nervous system. When the cord is cut across, two substances are readily 

 distinguishable, a central, roughly H-shaped area of gray matter surrounding 

 the canal, and a border of white matter around this (Fig. 16.16). In the gray 

 matter there are many cell bodies, but the white matter consists of great 



PRAY AISD WHITE f^ATTER OF SPmA.L CORD 



Post<2PlOP 



Post. Hoprv 



Gfcxy 



A.n.t. Koprx 



Cen.-tra.l cartal 

 corn.rru.^'i u. r»« 



WKlte 

 ■m.ci.tt«t> 



fiber 

 Ant. Kopn. cell 



rTu.clcu.'o of 

 rKzarosUoL cell 



r^«i?ve fiber* 

 r^ycUn. sK«o.tK 



Fig. 16.16. Cross section of the human spinal cord showing the gray matter 

 containing nerve cell bodies and their fibers, and the white matter containing 

 their fibers only. In the brain the gray matter is outside and the white within. The 

 central canal is continuous with cavities (ventricles) of the brain. (Courtesy, Ham: 

 Histology, ed. 2. Philadelphia, J. B. Lippincott Co., 1953.) 



