198 



PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



man the data, while not very complete, indicate that the paralysis 



does not pass off completely with 

 time. There may be a fair amount 

 of return of function, but a certain 

 "irreducible minimum" of paral- 

 ysis persists, especially in those 

 muscles which carry out the more 

 specialized movements. * The 

 more complete observations made 

 on the anthropoidsf indicate, on 

 the contrary, that after ablations 

 of considerable extent in the limb 

 areas the initial paralysis soon dis- 

 appears almost completely, and 

 when a second operation is made 

 on the corresponcUng area in the 

 other hemisphere, there is no re- 

 crudescence of paresis in the limb 

 which had recovered from the first 

 operation. The way in which this 

 vicarious substitution is effected is 

 not definitely known. In the dog 

 and animals still lower in the scale 

 of development one thinks of the 

 rubrospinal tract and its connec- 

 tions with the basal ganglion of 

 the cerebrum as offering a mech- 

 anism, in addition to the pyra- 

 midal system, for carrying out the 

 highly co-ordinated movements of 

 locomotion, etc., but the exact dis- 

 tribution of function between the 

 pyramidal (cortical) and the sub- 

 cortical motor systems is not un- 

 derstood. 

 The Crossed Control of the Muscles and Bilateral Represen- 

 tation in the Cortex. — It has been known from very ancient times 

 that an injury to the brain on one side is accmpanied by a paral- 

 ysis of voluntary movement on the other side of the body, a condi- 

 tion known as hemiplegia. The facts given above regarding the 

 origin and course of the pyramidal system of fibers explain the 



representing 



Fig. 89.— Schema 

 the course of the fibers of the pyra 

 midal system: 1, Fibers to the nuclei of 

 the cranial nerve ; 2, uncrossed fibers 

 to the lateral pyramidal fasciculus : 

 3, fibers to the anterior pyramidal 

 fasciculus crossing in the cord ; 4 and 

 5, fibers that cross in the pyramidal 

 decussation to make the lateral 

 pyramidal tract of the opposite side. 



* Frazier and Ingham, "Trans. American Neurological Association," 1919. 

 t Leyton and Sherrington, Loc. cit. 



