1244 AN AMERICA X TEXT- HOOK OF PHYSIOLOGY. 



passage of impulses from the point of stimulation toward the infracortical 

 structures. If, in addition, a cut be made below the cortex and parallel with 

 its surface, then stimulation of the cortex above this section is ineffective, thus 

 indicating that the impulses pass from the cortex directly into the substance 

 of the hemisphere along certain nerve-tracts, which by this operation were 

 sectioned. Further, if the bit of cortex thus separated from the underlying 

 white substance be removed and the faradic current be applied to the white 

 substance beneath, a reaction of the same type and involving the same mus- 

 cles can be obtained, although it differs from that to be gotten from the cor- 

 tex itself, in the first place by being less co-ordinated, in the second by con- 

 tinuing only so long as the stimulus lasts, and in the third place by giving 

 rise to less intense electrical changes connected with the passing impulse. 

 By careful exploration the bundle of fibres which is thus picked out can be 

 followed, as the brain substance is cut away, through the internal capsule and 

 the cerebral peduncles. 



These facts taken together lead to the conclusion that when the cortex is 

 stimulated the impulses concerned in producing the muscular contractions 

 traverse cell-bodies at the point of stimulation, and are transmitted thence 

 through the underlying fibres. AVe shall see later that this direct course 

 probably does not represent the sole pathway for these impulses. 



Course of the Descending Impulses. — The course of the impulses is 

 next inferred from the relation between the removal of different parts of the 

 cortex and the consequent secondary degenerations throughout the length of 

 the central nervous system. When the part of the cortex removed is taken 

 from the motor area then the degeneration occurs in the internal capsule and 

 in the callosum. The path of the fibres forming outgrowths of the cortical 

 cells can be followed thence through the crusta and pyramids to the spinal 

 cord. 



After removal of the motor region of one cerebral hemisphere the degen- 

 eration is mainly in the internal capsule and crusta of the same side, though 

 by way of fibres crossing in the callosum it may be traced to the other side 

 also. At the decussation of the pyramids the fibres occupying the internal 

 capsule of the same side as the lesion for the most part cross the middle line. 

 The portion which remains uncrossed passes as the direct pyramidal tract of 

 the ventral columns in man, while the crossed bundle, which is much the 

 larger, lies in the dorsolateral field of the lateral column, forming the crossed 

 pyramidal tract. Since the observations of Pitres 1 in 1881-82 evidence has 

 been accumulating to show that in man a lesion of the motor cortex of one 

 cerebral hemisphere is followed by a degeneration of the crossed pyramidal 

 tract on both sides of the cord. Of course, the degeneration in the hetero- 

 lateral tract is much the larger of the two. That the fibres degenerating in 

 the homolateral tract remain on the same side throughout their entire course 

 is shown by the physiological experiments of Wcrtheimer and Lapage 2 on 



1 ProgreS medieale, Paris, 1882, x. 528. 



1 Archives de Physiologic, 1897, No. 1, p. 168. 



