DIRECTION OF IMPULSES IN REFLEX ACTIONS. 821 



is the reverse of the view that since Winslow and Duchenne has been 

 a common doctrine concerning muscular co-ordination. It controverts 

 an argument adduced for the view that the limb movement evoked by 

 excitation of an efferent root represents a highly co-ordinate functional 

 synergism. 1 The spinal reflex in its intraspinal irradiation develops a 

 combined movement and synthesises a muscular harmony. 



vi. It follows almost as a corollary from this, and from the rule of 

 spatial proximity, that the spinal reflex movement elicitable in and from 

 any one spinal region, will exhibit much uniformity despite considerable 

 variety of the locus of incidence of the exciting stimulus. Approx- 

 imately the same movement, e.g. in the hind-limb flexion of the three 

 great joints, will result, whatever piece of the limb surface be irritated. 

 The seat of incidence of the stimulus will only influence the character 

 of the general movement executed by the limb musculature, in so far 

 that the flexion will tend to predominantly occur at that joint the 

 flexor muscles of which are innervated by motor cells segmen tally near 

 to the entrance of the afferent fibres from the particular piece of skin 

 the seat of application of the stimulus. 



Eegarding the course of irradiation in long intraspinal reflexes, 

 namely, those spinal reflexes that initiated from one of the above- 

 mentioned spinal regions spread over into others, Pfliiger, in 1853, 

 formulated four " laws." 2 He argued chiefly from a careful collation of 

 the reports of a number of clinical cases. These " laws " have for many 

 years been widely accepted. They are stated as follows : — 



1. The law of homonymous conduction for unilateral reflexes. If a 

 stimulus applied to a sensory nerve provokes muscular movements solely 

 on one side of the body, that movement occurs under all circumstances 

 and without exception on the same side of the body as the seat of 

 application of the stimulus. 



If, as is clear from the context in the original paper, by movement on the 

 same side be meant contraction of muscles on the same side, this statement 

 does not in reality very completely express the facts. It is in part an outcome 

 of the rule of spatial proximity, but certain cases which conform to the latter 

 yet offer striking exception to the former ; for instance, when the tail is 

 touched on one side, it very frequently is moved towards the opposite, and this 

 in a great number of classes, from the fish to the mammal inclusive. 



2. The law of bilateral symmetry of the reflex action. When the 

 change produced in the central organ by excitation of a sensory nerve 

 has already evoked unilateral reflex, it, if it spreads farther, excites in the 

 contralateral half of the cord only those motor mechanisms which are 

 symmetrical with those already excited in the homonymous half of the 

 cord. This statement, although true of a number of instances, fails to 

 conform with fact in many, even perhaps the majority. 



The important crossed reflex from the hind-limb of the bird and mammal does 

 not conform to it ; so similarly with the fore-limb. The asymmetry of the 

 crossed reflexes of the limbs is important, because probably connected with the 

 fundamental co-ordination of muscles for progression. Again, the wag reflex 

 of the tail, and a reflex I have called the " torticollis reflex " (cervical region), 

 afford important exceptions to the " law." And many other exceptions can be 



terrier and Yeo, Proc. Roy. Soc. London, 1881, vol. xxx. ; P. Bert and Marcacci, 

 Sperimentale, Firenze, October 1881 ; J. R. Russell, Proc. Roy. Soc. London, 1892, vol. li. 

 Ferrier has more recently (Brain, London, 1893) much modified his conclusions on this point. 



- " Die sensorische Funktionen d. Riickeinnarkes," Berlin, 1853. 



