522 HANDBOOK OF PHYSIOLOGY. 



In the internal capsule the fibres which pass onwards and downwards 

 to the pyramidal tracts of the spinal cord do not occupy more than a 

 small section, namely, that part known as the knee, and the anterior 

 two-thirds of the posterior segment (Fig. 360). In this district the 

 fibres for the face, arm, and leg, are in this relation: those for the face 

 and tongue are just at the knee, and below or behind them come first the 

 fibres for the arm and then those for the leg. The posterior third of the 

 posterior segment is occupied by the sensory fibres. 



Following the fibres downwards from the internal capsule it is found 

 that those which are motor in function descend in the crusta of the cms 

 on either side, where they are collected into the upper part of the mid- 

 dle third, and that they then pass through the pons to form the anterior 

 pyramids of the medulla. The fibres then either decussate in the 

 middle line, passing over to the opposite side to become the lateral or 

 crossed pyramidal tract of the lateral column of the cord, or remain as 

 the direct pyramidal tract of the anterior column on either side of the 

 anterior fissure. The direct pyramidal tracts, it will be remembered, 

 decussate by degrees in the cord. 



This pathway of the pyramidal fibres is demonstrated by their degen- 

 eration when any lesion separates the fibres from their corresponding 

 cortical cells, as, for example, a hemorrhage into the corpus striatum of 

 sufficient extent but the interruption may take place anywhere in the 

 whole course of the tract. If the whole of these fibres on one side is 

 destroyed transversely, above the decussation, hemiplegia of the opposite 

 side, more or less complete, results. The idea which was formerly held, 

 that some of these fibres pass through the corpus striatum does not 

 appear to be supported by sufficient evidence. They have an interrupted 

 course. The reason why a hemorrhage into the corpus striatum pro- 

 duces hemiplegia appears to be because of the almost certain pressure 

 which such a lesion exerts upon the fibres of the internal capsule. 



Sensory paths in the brain. The knowledge which we possess of the 

 distribution of the sensory fibres in the brain is not nearly so definite as 

 that which has been obtained of the motor tracts. As we have seen, the 

 course of the sensory fibres even in the cord is not by any means com- 

 pletely understood. Supposing such fibres to be contained chiefly in the 

 anterior part of the lateral columns and in the posterior columns of the 

 cord, having previously crossed over to the opposite side of the cord to 

 that from whence they came, they probably proceed in the posterior half 

 of the medulla, chiefly in the formatio reticularis, and in the correspond- 

 ing part of the pons, beneath the corpora quadrigemina to the tegmen- 

 tum of the crus. In this they pass above the locus niger, and enter the 

 posterior third of the posterior limb of the internal capsule (sensory 

 crossway}. From this district the fibres pass on into the white matter 

 of the brain and probably extend into the so-called motorial areas already 



