THE PATTERNING OF SKILLED MOVEMENTS 



1687 



tion, in contrast to the physiological and direct 

 access to the spinal keyboard available through 

 reflex paths; c) the more or less indirect nature of the 

 peripheral criteria of this central activity (with its 

 possible remodeling at the level of the spinal key- 

 board); and finally d) the important transformations 

 introduced by the progress of phylogenesis in the 

 organization of these structures which render the 

 results obtained from one species difficult to com- 

 pare with those from another. 



It seems indisputable that the response most 

 easily obtained by applying the stimulating elec- 

 trodes to the motor cortex is that of a muscular group 

 which is often synergistically activated. The 'physio- 

 logically minded' neurologist (128) may then rightly 

 speak of a functional organization of this cortical 

 region. However, proper experimental procedures 

 may reveal the true anatomical arrangement of the 

 cortical motor keyboard. All evidence now available, 

 obtained both by localized stimulation and by de- 

 struction, indicates clearly that isolated muscles (22) 

 or even parts of muscles (97) are 'represented' in the 

 precentral gyrus. 



The overlapping of the various muscular areas is 

 accentuated when one proceeds from the distal lo 

 the proximal parts of the limbs. The localization of 

 the distal muscles of the forelimbs becomes more and 

 more precise when we progress from the cat to the 

 monkey and then to man. The enormous importance 

 assumed by the 'representation' of the hand and of 

 the fingers in man (96) gives us an inkling of the 

 functional significance which must lie attributed to 

 this topographical arrangement for the cortical con- 

 trol of discrete movements of the fingers. 



Concerning the anatomical basis of this functional 

 representation, it is particularly worth while to 

 search for the architectural transformation which 

 underlies such a phylogenetic advance. The clear 

 individuation in the agranular cortex of man of an 

 area 47 (area gigantocellularis) penetrating in tin- 

 depth of the fissure of Rolando, with the identifica- 

 tion of the giant pyramidal cells of Betz in the fifth 

 layer, very early attracted attention to this vcrv 

 particular aspect of cortical architecture (122). 



These cells increase in number and size, and 

 diminish in density from the anthropoids to man. 

 Numbering 34,000 in the human area 4, according 

 to Lassek (69), they constitute a fine anatomically 

 differentiated keyboard. From the functional point 

 of view, such cells bear direct comparison with the 

 motoneurons of the spinal keyboard. They appear as 

 foci of convergence and of integration for facilitating 



and inhibiting influences of various origins. They are 

 associated, like the lower motoneurons of the spinal 

 level, with a complex arrangement of internuncial 

 cells which contributes to the modulation of their 

 excitatory state. Thus the patterned activation of the 

 elements of the upper motoneuron keyboard, like 

 that of the spinal structures, depends upon a complex 

 integrative operation. 



We now have to specify the paths and the means 

 by which the cortical commands act upon the spinal 

 keyboard. 



The Corticumotoneural Trad 



The part played by the pyramidal tract in the 

 evolutionary perfecting of the motor system marked 

 it very early as the chief executive pathway bv 

 which each cortical motor area controls the muscula- 

 ture of the opposite side of the body. Our knowledge 

 of the pyramidal system, which Bernhard has re- 

 cently suggested might better be named the 'corlico- 

 motoneur.il tract,' has advanced considerably in the 

 course of the last decide (69). The simplicity of the 

 primitive conceptions has undergone important re- 

 visions (see Chapters XXXIII and XXXIY on the 

 pyramidal tract in this Handbook). 



Particularly, the identification of a group of special 

 fibers in the very interior of this sv stem deserves 

 deeper consideration. It now seems established that 

 the contingent of thick myelinated fibers, which 

 constitute about three per cent of the fibers of the 

 pyramidal tract (69), is formed by the axons arising 

 from the giant pyramid. ll cells of the precentral 

 gyrus. They, therefore, form the most rapid associa- 

 tion path between the motor cortex and the spinal 

 kev hoard. 



The electrophysiological studies of Lloyd (79) have 

 for the first lime precisely described the spinal or- 

 ganization of the pyramidal projections in the cat. 

 In agreement with the histological data of Szentagotai 

 (115), they support the concept of the obligatory 

 relaying of corticospinal messages in the internuncial 

 sv stems of the spinal cord. The absence of a direct 

 control of the spinal kev board by the cortical key- 

 board was at first rather disappointing. It appeared, 

 indeed, to limit greatly the authority of cortical 

 control over the spinal machinery. However, new 

 and precise findings of great importance have been 

 obtained concerning the character of this liaison in 

 the monkey. Thanks to appropriate techniques of 

 stimulation, Bernhard and his collaborators (12) have 

 recentlv established the existence in the monkey of a 



