628 THE BKAIN. 



constituting the front ending of the tegmentum, we may arrange a large part 

 of the brain into two main regions into a tegmental region stretching from 

 the optic thalamus through the dorsal portion of the pons to the dorsal por- 

 tion of the bulb, and into a region, which we may call the pedal region, 

 stretching from the internal capsule through the ventral portion of the pons 

 to the ventral portion of the bulb. 



The fibres of the brain, as a whole, may be broadly classified into longi- 

 tudinal tracts connecting parts of the brain with succeeding parts and into 

 transverse or commissural tracts between one lateral half and the other, and 

 into tracts connected with the several cranial nerves. Taking the longitu- 

 dinal fibres first, we may in accordance with the division just explained into 

 a pedal 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 hemi- 

 spheres. 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 tem- 

 poral (Figs. 139, 140, 144). 



LONGITUDINAL FIBRES OF THE PEDAL SYSTEM. 



545. The pyramidal tract. We have already ( 488) 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 gray matter 

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

 their share to the central white matter of the hemisphere, converge (Figs. 

 145, 146) to form part of the internal capsule, namely, that part which in a 

 horizontal section (Fig. 144, 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 (Fig. 139, 146), and so contribute to 

 form the beginning of the crus cerebri. In thus converging to take up their 

 position in the capsule ancl in their further passage to the crus the fibres 

 follow a course of somewhat complicated curvature. As we trace the cap- 

 sule from more dorsal to more ventral levels, we find it continually changing 

 in form ; the exact shape of the capsule shown in Fig. 144 only holds good 

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

 shown in Fig. 138 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 sections, 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 

 capsule, are continually changing their direction as they pass through the 

 capsule. Moreover, while the fibres from the different parts of the " motor 

 area " assume definite positions in relation to each other as they pass into 



