842 THE SPINAL CORD. 



any other change in the conditions than the intercurrent stimulation of 

 an adjacent root. Again, when a bilateral reflex is started by excitation of 

 an area of skin or a nerve trunk, and, .as often happens, the contralateral 

 reflex is the lesser, the reflex at first obtained is unilateral and homonymous, 

 but on then proceeding to excite the corresponding skin or nerve trunk of the 

 opposite side, the reflex movement evoked is at once bilateral and stronger 

 contralaterally. The stimulation of the first side has by its homonymous 

 action "canalised" the contralateral part of the mechanism replying to the 

 second side. 



An instructive instance is yielded by the following. Excitation of the 

 afferent root of either the eighth or seventh cervical nerve evokes in the spinal 

 mammal sometimes flexion at elbow, sometimes, on the contrary, extension. 

 Excitation of the afferent root of the first thoracic nerve evokes almost invariably 

 extension at elbow ; excitation of the afferent root of the sixth cervical evokes 

 almost invariably flexion at elbow. If excitation of the afferent root of the 

 eighth be preceded by excitation of the root of the first thoracic, the movement 

 at elbow evoked by the eighth is almost always extension, and similarly though 

 not so regularly by the seventh. If, on the contrary, the excitation of the 

 seventh be preceded by excitation, not of the first thoracic, but of the sixth 

 cervical, the movement it evokes at the elbow is almost sure to be flexion ; and 

 similarly though not so regularly with excitation of the eighth. 



There are many sequences met in experimenting on the cord 

 that the example just quoted typifies. Noteworthy is that the 

 flexors and extensors were never seen to be simultaneously thrown into 

 contraction. I would express the general result by saying that sensory 

 stimuli and reflex reactions which tend, when separate in time, to 

 excite the same group of motor cells, canalise for and tend, when 

 contemporary, to reinforce one another ; those which tend, when separate, 

 to excite different but inter-related groups of motor cells, tend, when 

 contemporary, to inhibit one another. 1 



The incitement to activity of functional group A of motor units seems 

 colligate with inhibition of the functional group B, whose muscles are 

 antagonistic to those innervated by A. By what mechanism this is ensured is 

 not clear. Group A may have a path not only to the muscles but to the nerve cells 

 of group B, by which it can depress the activity of the latter. Or each spinal 

 path that embouches into A may also embouch mediately or immediately into 

 B, and while exciting A inhibit B. This latter does not necessarily imply 

 two different kinds of disturbance in the paths opening upon A and B 

 respectively ; a difference between the condition obtaining at the moment in 

 A and B motor units may determine a different reaction in them under 

 an external influence which falls similarly on both. Whatever the detailed 

 explanation, it is certain that co-ordination is a character common to all reflex 

 actions from the unpoisoned spinal cord, and that reciprocal innervation largely 

 expresses that co-ordination. 



The movements that are in the spinal animal at various times 

 elicited from one and the same area, and even same spot, of skin, vary 

 considerably, and even at brief intervals. No phenomenon more often 



1 I am aware this differs from views expressed by Eckhard, Goltz, SchifF, Herzen, 

 Wundt, and others ; but see the suggestions of Descartes, " De homine," Amsterdam, 1678, 

 p. 34 ; of C. Bell, in a footnote, " Physiology of the Human Body," London, 1826, vol. iii. ; 

 also "Remarks on Respiratory Co-ordination," by S. J. Meltzer, Arch. f. Physiol, Leipzig, 

 1883 ; and New York Med. Journ., 1899 ; and my notes on "Reciprocal Innervation," in 

 Proc. Roy. Soc. London, vol. Hi. et seq., 1893-1898, and elsewhere. The suggestions of 

 Descartes and Bell proposed, however, peripheral inhibition, not central. 



