

CHAP, ii.] THE BRAIN. 749 



tion of areas for the several movements is carried out on the same 

 plan as in the monkey (we are purposely confining ourselves now 

 to the results of artificial stimulation) ; and moreover, justify the 

 conclusion, which a priori reasons would lead us to adopt, that 

 in man the differentiation is advanced still farther than in the 

 monkey. 



Thus when we survey a series of brains in succession, from 

 the more lowly frog, through the bird, the rabbit, the dog, and 

 other lower mammals up to the monkey, the anthropoid ape, and 

 so to man himself, we find an increasing differentiation of the 

 cerebral cortex, by which certain areas of the cortex are brought 

 into special connection with certain skeletal or other muscles in 

 such a way that stimulation of a particular portion of the grey 

 matter gives rise to a particular movement and to that alone. 



486. Before proceeding further, it will be perhaps advan- 

 tageous to call to mind some of the important features of the great 

 strand of fibres known as the pyramidal tract. These fibres start 

 from the cerebral cortex of the motor region which we are study- 

 ing ; they are the axis cylinder processes of some of the large 

 pyramidal cells which are so conspicuous in this region of the 

 cortex. Passing downwards from the cortex through the white 

 matter of the hemisphere and the corona radiata they are gath- 

 ered up into the internal capsule between the nucleus lenticularis 

 on the one side and the nucleus caudatus with the optic thalamus 

 on the other side (Figs. 125, 126, 127). The internal capsule has 

 the form of a fan, the handle of which passes into the crus cerebri, 

 while the expansion stretches into the hemisphere ; it is further 

 bent into a rounded angle (Fig. 125, G), the 4 knee,' which sepa- 

 rates a front from a hind limb. The fibres of the pyramidal tract 

 occupy the knee, a small adjoining portion of the front limb and 

 a large part of the hind limb ; but it must be remembered that 

 when we examine the internal capsule by horizontal sections 

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

 find that the knee shifts in 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. When the capsule 

 has thus passed into the crus, or rather into the pes of the crus, 

 the pyramidal tract is found occupying the median region of 

 the pes (Fig. 128). Farther backward the fibres of the tract are 

 found running in the pons (Fig. 129, 130, 131), in strands in- 

 terwoven with the transverse fibres of that structure, and then 

 issuing from the pons are found concentrated again into the an- 

 terior pyramids of the bulb (Fig. 132). At the decussation of 

 the pyramids, while most of the fibres cross over to form the 

 crossed pyramidal tract of the spinal cord some are continued 

 on as the direct pyramidal tract, which however speedily dimin- 



