070 THE BRAIN. 



impulses accompanied by " currents of action " pass downward along the 

 fibres of the pyramidal tract. 



The results of stimulating the fibres of the tract in their course through 

 the corona radiata and the internal capsule, and the results obtained by 

 studying the degenerations following upon injury to or removal of the several 

 parts of the cortical motor region, agree in marking out the paths taken by 

 the several constituents of the tract through the central white riiatter of the 

 hemisphere, the corona radiata, and the capsule. Comparing Figs. 149, 150, 

 with Figs. 144, 145, and 146, it will be seen that the portions of the tract 

 destined for the cranial nerves, and so for the movements of the eyes, the 

 mouth, face, tongue, pharynx, and larynx, starting from the ventral parts of 

 the more frontal district of the motor region, take up their position at the 

 knee of the internal capsule ; and the portion destined for those upper 

 cervical nerves which carry out movements of the head through the muscles 

 of the neck, starting from the extreme frontal and dorsal parts of the area, 

 is also apparently directed to the knee of the capsule. The rest of the tract, 

 starting from the part of the area lying at once behind and mesial to the 

 above, occupies in the capsule a position posterior to them in the hind limb 

 of the capsule ; and it will be observed that the tract for the fore limb which 

 begins on the lateral surface of the tracts for the trunk and hind limb, shifts 

 its course in relation to theirs, so that in the capsule it is front of them, 

 not lateral to them. It may further be observed that while in the tracts for 

 the trunk and hind, limb the same fore-and-aft order which obtains on the 

 surface is reproduced in the capsule, even apparently to the strange prece- 

 dence of the ankle over the knee, the order of the several elements in the 

 fore-limb tract which is lateral on the surface becomes regularly fore and aft 

 in the capsule. In the capsule the several elements are arranged in a linear 

 order, corresponding broadly to that of the distribution of the muscles along 

 the longitudinal axis of the body; on the cortex they are disposed in an 

 order the cause of which is at present not very clear, but which is probably 

 determined by the respective relations of the several parts of the motor 

 region to the functional activity of the other parts of the cortex. In the 

 shifting from the one order to the other, the several constituent fibres, as we 

 have said, describe a somewhat peculiar course ; and when we remember, as 

 stated in 545, that the order shown in Fig. 144 is only the order obtaining 

 at one particular level of the capsule, and that from the dorsal beginnings 

 of the capsule in the corona radiata to its ventral end in the pes the capsule 

 is continually changing in form, and its fibres therefore continually shifting 

 their relations to each other, the whole course of the several fibres of the 

 tract from their origin in the cortex until they are gathered up into the 

 central portion of the pes (Fig. 137, Py.) must be a very complicated one. 



When the .area of one hemisphere is stimulated, the movement which 

 results is in most cases seen on the other side of the body, and on that other 

 side alone. Thus when the area for the fore limb on the left hemisphere 

 is stimulated it is the right fore limb which is moved. This is in accordance 

 with what we have learned of the pyramidal tract and its ultimate entire 

 decussation before it reaches the motor nerves, the decussation either occur- 

 ring massively as in the case of the crossed pyramidal tract, or in a more 

 scattered manner along the upper part of the spinal cord in the case of the 

 direct pyramidal tract ; and, as we have seen, there is a similar decussation 

 for such part of the pyramidal tract as is connected with the cranial nerves 

 above the decussation of the pyramids. Except in the case of certain areas 

 for movements naturally bilateral, of which we shall speak presently, the 

 movement is normally on the crossed side, and on the crossed side only. 

 Under abnormal conditions, however, the limb on the other side that is, of 



