174 



THE HUMAN BODY 



Auditory impulses enter the medulla by way of the auditory 

 nerves. The axons of the nerves themselves terminate in nuclei 

 of the medulla, the auditory nuclei; new neurons continue the 

 path thence across the mid-line of the medulla and forward into 

 the midbrain terminating in the internal geniculate nuclei and 

 the inferior colliculi. From these nuclei a third set of neurons 

 continue the path to the cerebrum. 



General Structure of the Cerebrum. This organ consists, as 

 previously stated, of an outer surface of gray matter, two milli- 



Entering posterior 

 root 



Lissauer's tract 



anterior root 



FIG. 65. Diagrammatic transverse section of the spinal cord showing the con- 

 duction paths. (Cunningham.) 



meters thick, overlying a mass of white matter; the whole held 

 together by neuroglia and connective tissue, and mounted upon 

 the midbrain as upon a stalk. Because of the convoluted surface 

 of the cerebrum the total amount of superficial gray matter is 

 much greater than it would be if the cerebrum were smooth. This 

 layer of gray matter is the region wherein occur those special 

 activities which set the cerebrum above the rest of the nervous 

 system. It is called the cortex cerebri, or for convenience simply 

 the cortex. 



Structure of the Cortex. The cortex cerebri consists for the 

 most part of neurons with small cell-bodies having much branched 



