CHAP. IL] THE BRAIN. 987 



and a tegmental region, consider these as forming on the one 

 hand a pedal, and on the other hand a tegmental system. 



Both systems begin as we shall see in the cortex of the 

 cerebral hemispheres. We shall have to deal with the topography 

 of the cortex later on> but may here say that the first broad 

 division of the whole surface of a hemisphere is into four main 

 regions: frontal, parietal, occipital and temporal (Figs. 116, 117, 

 121). 



Longitudinal fibres of the Pedal System. 



632. The pyramidal tract. We have already (575) said 

 that the pyramidal tract of the spinal cord may be traced to a 

 particular region of the cerebral cortex. We shall study the 

 details of this region, which is often spoken of as the " motor area" 

 later on, but may here say that broadly speaking it is parietal in 

 position and corresponds to the parts of the cortex gathered round 

 the fissure of Rolando. Fibres passing from the grey matter of 

 the cortex of this region to the white matter below, and so con- 

 tributing their share to the central white matter of the hemisphere, 

 converge (Figs. 122, 123) to form part of the internal capsule, 

 namely that part which in a horizontal section (Fig. 121, Eye to 

 Dig} occupies the knee and stretches for more than half, or nearly 

 two-thirds, along the hind limb of the capsule, between the optic 

 thalamus on the inside and the nucleus lenticularis on the outside. 

 From the knee and hind limb of the capsule they pass by the 

 side of and ventral to the optic thalamus (Figs. 116, 123), and 

 so contribute to form the beginning of the crus cerebri. In thus 

 converging to take up their position in the capsule and in their 

 further passage to the crus the fibres follow a course of somewhat 

 complicated curvature. As we trace the capsule from more dorsal 

 to more ventral levels, we find it continually changing in form ; 

 the exact shape of the capsule shewn in Fig. 121 only holds good 

 for the level at which the section was taken ; it differs somewhat 

 from that shewn in Fig. 115 taken at a slightly different level, and 

 sections still more dorsal or still more ventral would present still 

 greater differences. When we examine a series of horizontal sec- 

 tions, taken in succession from the dorsal to the ventral regions, 

 we find that the knee shifts its position and changes in the width 

 of its angle, that the two limbs vary in direction in size and in 

 shape, and that at last the bent flattened capsule passes into the 

 more or less rounded crus by the rapid disappearance of the 

 fore limb, and the consequent extinction of the angle ; so that in 

 one sense it is the hind limb which becomes the crus, and the 

 fibres of the fore limb may be said to pass into the crus through 

 the ventral portion of the hind limb. Hence it is obvious that 

 the fibres of the pyramidal tract, like the other fibres of the 



