68 



THE NERVOUS SYSTEM 



FIG. 39 



the internal capsule. This is simply the laterally bent and flattened sheaf 

 of fibers from the cortex where they pass between the lenticular nucleus 

 on the outside and the corpora striata and optic thalami within, as the 

 illustration shows. Similarly, the mysterious corpus callosum is a thick, 

 hard bundle of fibers connecting intimately the two hemispheres. Not 



infrequently, however, it is absent 

 altogether without obvious func- 

 tional defect in the individual. In 

 an homologous way the lower or 

 temporal convolutions exhibit con- 

 verging fans of fibers passing in- 

 ward and upward into the crura 

 of the brain. What sorts of im- 

 pulses, and in what directions, 

 these various fibers conduct will 

 be somewhat better understood 

 when the functions of the cortex 

 cerebri and the nuclei which they 

 serve have been described. 



The Cerebral Cortex. This is 

 the outermost or bounding layer 

 of the hemispheres, and is from 

 2 to 4 mm. in thickness in differ- 

 ent parts. The area of the cortex 

 is increased two or three times at 

 least by the numerous sulci and 

 fissures, 2 or 3 cm. deep, in the 

 surface of the hemispheres. The 

 cortex dips down into these sulci 

 in all cases and lines them on 

 both sides and on the bottom. 

 Indeed, these sulci appear to be 

 present for the purpose of increas- 

 ing the area of the cortex, and they 

 are more numerous and deeper 

 the more highly evolved and the 

 more skilful and intelligent the 

 animal. The cortex is composed 

 of various layers of nerve-cells 

 and the fibers connecting them. 

 A recent estimate of the number of 

 these cells in the human cortex is 9,200,000,000; other estimates state 

 that the number of these cells is several times as small. They weigh, 

 however, only about seventeen grams. The rounded masses of brain 

 between the sulci and fissures are the gyri or convolutions, and each of 

 these has a name corresponding to its position and shape. 



Aside from the great longitudinal fissure separating the hemispheres 



Section of human cerebral cortex, to suggest 

 especially the immense complexity of the neuronal 

 relations (methods of Weigert and Golgi): C. Z., 

 clear zone having no nerve fibers; M . P., Exner's 

 plexus in the molecular layer; A. sir., ambiguous 

 cell stratum; Subm. P., submolecular plexus; 

 Gt. P. P., great pyramidal plexus; Pol. P., poly- 

 morphic plexus; W., white matter. (Andriezen.) 



