820 THE SPINAL CORD. 



amples of intraspinal irradiation which they furnish, it is possible to make 

 certain general statements. 1 



i. Broadly speaking, the degree of reflex spinal intimacy between 

 afferent and efferent spinal roots varies directly as their segmental prox- 

 imity. Thus excitation of the central side of a severed thoracic root, 

 e.g. seventh, evokes with especial ease contraction of muscles or parts of 

 muscles innervated by the corresponding motor roots, and next easily 

 muscles innervated by the next adjacent motor roots. The spread of 

 short spinal reflexes in many instances seems to be rather easier 

 tailward than headward. This may be related with the oblique 

 correlation that so largely holds between the distribution of the 

 afferent root in the skin, and the distribution of the efferent root 

 in the underlying muscles. 2 



ii. Taken generally, for each afferent root there exists in immediate 

 proximity to its own place of entrance in the cord (e.g. in its own segment) 

 a reflex motor path of as low resistance as any open to it anywhere. Further, 

 in response to excitation of even approximately minimal intensity, a single 

 afferent root, or a single filament of a single root, evokes a spinal dis- 

 charge of centrifugal impulses, which tends to occur not only through 

 its own efferent root but through more than one efferent root, i.e. is 

 plurisegmental. And this holds especially true in the limb regions. 

 In the limb region the nerve root is therefore a morphological aggregate 

 of nerve fibres, rather than a functionally determined assortment of 

 impulse paths. The formation of functional collections of nerve paths 

 (peripheral nerve trunks) out of morphological collections (nerve roots) 

 seems to be the meaning of the limb plexuses. 



iii. Motor mechanisms for the skeletal musculature lying in the same 

 region of the cord, and in the self-same spinal segment, exhibit markedly 

 unequal accessibility to the local afferent channels as judged by pressor effects? 

 For example, if the primary phase only of the reflex movement be con- 

 sidered, the flexors of the homonymous knee and the extensors of the 

 contralateral are, in most animals, much more accessible than the exten- 

 sors of the homonymous and the flexors of the contralateral. Inas- 

 much as at many joints the flexors and extensors are both innervated by 

 motor fibres contained in one and the same efferent root, it follows that 

 the reflex movement obtained from excitation of an afferent root in 

 many cases is quite dissimilar from the movement obtained by excita- 

 tion of the corresponding efferent root, 4 in spite of the rule of segmental 

 proximity. 



iv. Where a spinal reflex discharge is prolonged, it usually involves 

 antagonistic sets of motor cells alternately ; it is in each of these several 

 phases that statement iii. holds good, not in the whole discharge taken 

 collectively as a single discharge. It then only applies when, as in 

 very many instances, the reflex movement is not prolonged, and there 

 is no secondary reversal. 



v. The groups of motor nerve cells contemporaneously discharged by 

 spinal reflex action innervate synergic and not antergic muscles. This 



1 Sherrington, Phil. Trans., London, 1897. 



2 Sherrington, ibid. ; W. Page May, ibid., 1896. 



3 It seems necessary to insert the qualification " pressor" before "effects" in the above 

 statement, because there is some evidence ("reciprocal innervation") that inhibitory 

 effects accompany the pressor effects, and it may be only in regard to pressor effect that 

 in any particular case the above statement holds. 



4 Sherrington, Phil. Trans., London, 1892; W. Page May, loc. cit., 1896. 



