Responses of Higher Animals: The Nervous System - 459 



Fig. 25-11. Brains and spinal 

 cords of man, the cat, and the 

 frog, showing the relative sizes 

 and proportionately greater 

 development of the brain in 

 man and the cat. (From The 

 Nervous System. Encyclopedia 

 Britannica Films, Inc.) 



mass of gray matter, consisting mainly of 

 nerve cell bodies, together with the dendrons 

 and axons that connect with the centrons. 

 The grayness of the central area is due to an 

 absence of myelin. This whitish lipoid ma- 

 terial invests the axons only after they emerge 

 from the gray matter and start running up- 

 ward or downward through the cord. Some 

 of the centrons, especially in the two ventral 

 horns of gray matter (Fig. 25-13), are the 

 cell bodies of motor neurons, which dispatch 



their axons directly out into the motor nerves; 

 but the other centrons belong to association 

 neurons, which lie entirely in the central 

 nervous system. 



The brain stem, or lower part of the brain, 

 has much the same structure as the spinal 

 cord, but the walls of the upper part of the 

 brain are greatly expanded, forming the so- 

 called higher centers. The most prominent 

 higher centers — in man and other mammals 

 — are the cerebrum and the cerebellum (Fig. 



Fig. 25-12. Median longitudinal sec- 

 tion of human brain. 



OLFACTORY LOBE 



