132 THE HUMAN BODY 



special part have come; for example, it was believed for a long 

 time that the axons mediating the sense of taste enter the brain 

 as part of the trigeminal nerve. It is now practically certain that 

 they enter instead by way of the facial and glossopharyngeal. 



White and Gray Matter. In preceding paragraphs the occur- 

 rence of white and gray matter in the central nervous system has 

 been mentioned. In the paragraph on myelin sheaths (p. 118) 

 the difference between them was described. It may be worth 

 while, for emphasis, to state again this difference before discussing 

 more specifically their distribution in the nervous system. White 

 matter consists of medullated axons, and is concerned function- 

 ally, therefore, with the conduction of impulses from point to 

 point. Gray matter consists of cell-bodies, dendrites, and parts of 

 axons, and in it and it alone are the synapses found over which 

 impulses pass across from one neuron to another. Gray matter, 

 therefore, is concerned with the distribution of nerve impulses 

 among the neurons. In it also, as we shall see, take place the 

 modifications which nerve impulses undergo during their passage 

 through the central nervous system. 



Most of the gray matter of the body is found in three special 

 regions. These are: (1) the gray columns of the spinal cord; 

 (2) a layer about 2 mm. (^ in.) thick over the entire outer surface 

 of the cerebral hemispheres, including the mesial surface of each, 

 and (3) a similar layer over the surface of the cerebellum. In 

 addition to these chief gray regions there are a number of small 

 masses of gray matter distributed in various parts of the body. 

 Some of these are imbedded in the brain; others are outside the 

 central nervous system altogether. Those within the central nerv- 

 ous system are known as nuclei,* those outside it as ganglia. 



The gray nuclei are found in the following regions: (1) The 

 base of the cerebrum; these are known as the basal nuclei and 

 include the optic thalami, the caudate, and the lenticular nuclei; 

 (2) the base of the cerebellum; here are several pairs of nuclei, 

 including the dentate nuclei; (3) the midbrain; here are several 

 small nuclei, the superior and inferior colliculi (corpora quadri- 

 gemina), the external and internal geniculate bodies, and the red 



* It must be understood that the term nucleus as applied to a mass of gray 

 nervous matter has an entirely different significance than when applied to a 

 part of a single cell. 



