THE PAEIETAL KEGION. 



663 



ing posterior central area becomes raised up into a prominent ridge, and a similar 

 ridge is formed immediately in front of it (Fig. 590) from the area which emits 

 the great efferent or motor tract to control all the motor nuclei upon the other 

 side of the brain and spinal medulla. As these ridges become raised up a 

 depression is left between them : this is the sulcus centralis. At first it consists 

 of two parts, a superior and an inferior (Fig. 590, r 2 and r 1 ) ; but as a rule these 

 become confluent later. 



The sulcus centralis in the adult takes an oblique course across the lateral 

 convex surface of the cerebral hemisphere, and intervening between the frontal 

 and parietal regions it forms the immediate posterior boundary of the motor area of 

 the cortex. Its upper end cuts the supero-medial border of the hemisphere a short 

 distance behind the mid-point between the frontal and occipital poles, whilst its 

 inferior end terminates above the middle of the posterior ramus of the lateral fissure. 

 Its superior extremity, as a rule, turns round the supero-medial border of the hemi- 

 sphere, and is then continued backwards for a short distance on the medial surface. 

 Although, in its general direction, the 

 sulcus is oblique, it is very far from being 

 straight. It takes a sinuous course 

 across the hemisphere. This is largely 

 due to the varying breadth of the motor 

 areas representing the lower limb, trunk, 

 upper limb, and head, respectively, which 

 are placed immediately in front of it. Ik interlocking 



When the sulcus centralis is widely opened 

 up, so that its bottom and its opposed sides 

 may be fully inspected, it will be seen that the 

 two bounding gyri are dovetailed into each 

 other by a number of interlocking gyri, which 

 do not appear on the surface (Fig. 591). Further, 

 two of these, placed on opposite sides of the 

 fissure, are frequently joined across the bottom 

 of the sulcus in the form of a sunken bridge of 

 connexion, which constitutes what is termed a 

 deep transitional gyrus. The continuity of the 

 sulcus is thus, to some extent, interrupted. 

 This condition is rendered interesting when 

 considered in connexion with the development 

 of the sulcus. The deep interlocking gyri in- 

 dicate a great exuberance of cortical growth in 

 this situation in the early stages of the develop- 

 ment of the sulcus ; and the presence of the deep 

 transitional gyrus is explained by the fact that the sulcus generally develops in two pieces, which 

 run into each other to form the continuous sulcus of the adult, viz., a part corresponding to the 

 inferior two-thirds, and a superior part, which represents the superior third and which appears 

 at a slightly later date. In certain very rare cases the sulcus centralis is found to remain double 

 throughout life, through a failure of its two pieces to unite. In such cases the deep transitional 

 gyrus, which is frequently seen at the bottom of the furrow, remains on the surface. Heschl, 

 who examined 2174 cerebral hemispheres, found this anomaly only six times ; Eberstaller met 

 with it twice in 200 brains. 



If a section is made at right angles to this sulcus in a fresh brain (Fig. 592), it 

 will be seen that its anterior (gyrus centralis anterior) and posterior (gyrus centralis 

 posterior) walls present a marked contrast the one to the other, and that the transi- 

 tion from the one type of cortex to the other takes place precisely at, or near to, 

 the bottom of the sulcus. The anterior wall is composed of thick (3 '5 to 4 mm.) 

 motor cortex thickly laden with medullary matter arranged in the form of three 

 or four pale bands with blurred edges and multitudes of fine pencils of fibres 

 passing to and fro between it and the white matter of the hemisphere. The 

 posterior wall is composed of thin (1-5 mm.) cortex containing two narrow and 

 sharply denned white lines. 



This sensory area forms little more than the posterior wall of the sulcus 

 centralis, and barely emerges upon the surface to form the posterior lip of the 

 sulcus (Fig. 581). Here it becomes continuous with a slightly thicker cortex with 



FIG. 591. SULCUS CENTRALIS FULLY OPENED UP, 

 so as to exhibit the interlocking gyri and deep 

 transitional gyrus within it. 



Motor cortex coloured red, sensory cortex blue. 



