THE CORTICAL GRAY MATTER. 177 



I. THE CORTICAL GRAY MATTER. 



The substantia corticalis consists of a thin envelope, the cortex 

 (or bark), which forms the surface of the hemispheres and incloses 

 the white medulla, the centrum semiovale. The cortex varies in 

 thickness from a line to a quarter of an inch. Thickest on the 

 surface of the gyrus, it grows thinner to the bottom of the sulci. 

 It is of a reddish, or yellowish-gray color depending on the richness 

 of the blood supply. By repeated observation of the symptoms 

 produced by definite brain lesions and by the anatomical and 

 physiological study of human and lower animal brains, both in 

 the embryonic and adult condition, the cortex has been mapped 

 out into certain definite functional areas (Figs. 54 and 55). Psychic 

 function undoubtedly is dependent upon the associated activity 

 of a number of cortical areas; but motor, common sensory and 

 special sensory regions have been outlined with considerable 

 exactness. 



Cortical or Cerebral Localization (Figs. 54 and 55). In 

 the following study of the cerebral cortex I shall use very exten-' 

 sively the recent work of Dr. Alfred W. Campbell, entitled, 

 " Histological Studies on the Localization of Cerebral Function," 

 Cambridge, England. In this epoch-making work we are shown 

 that certain cortical areas have a characteristic histological struc- 

 ture that distinguishes them from all other areas. This will be 

 referred to later under "cell and fiber lamination of the cortex," 

 but to appreciate this histological evidence of localization one 

 should thoroughly study the above work. 



Motor Area (Figs. 54, 55, 56 and 57). The emissive motor 

 area is situated in the anterior wall of the central sulcus, in the 

 posterior one-half of the gyrus centralis anterior and in that part 

 of the paracentral lobule immediately continuous with it. This 

 is the center for ordinary voluntary motion on the opposite side 

 of the body. Axones from this area descend to the nuclei of all 

 motor nerves. In lateral sclerosis there is degeneration and 

 disappearance of the giant pyramidal cells limited to this motor 

 area (Campbell). It is divided into four segments: the head 

 and neck, the arm, the trunk, and the leg, named from below up- 

 ward. The first extends to the inferior knee of the sulcus centralis 



