the Lumbo- Sacral Plexus of Macacus rhesus. 267 



intact, degeneration resulted in the muscles of corresponding func- 

 tion, and in these alone : those of opposite function showing no sign 

 of degeneration. 



The group of muscles supplied by any given nerve root occupy 

 both the anterior and posterior surfaces of the limb ;* in other words, 

 muscles whose unimpeded action would produce one movement are 

 represented in the same root as others whose action would produce a 

 movement diametrically opposite, e.g., the flexor and extensor muscles 

 of the ankle are represented in the same nerve root. In such com- 

 binations certain muscles are always more extensively represented 

 than others ; so that, with a current sufficiently strong to stimulate 

 all the fibres in a nerve root equally, certain muscles predominate in 

 their action over others. The 6th lumbar root contains fibres which 

 supply the flexors of the digits and fibres which supply the extensors, 

 and yet when the whole of the fibres in the nerve root are simul- 

 taneously and equally excited flexion of the digits is brought about 

 owing to the flexor muscles predominating over the extensors. This 

 predominance of one group over another does not always obtain, 

 however, as in the case of the ankle joint, where both the flexors and 

 extensors are supplied by nerve fibres derived from the 6th lumbar 

 root ; simultaneous and equal excitation of all the fibres contained 

 in this root causes the foot to be fixed at right angles at tha ankle, 

 neither the flexors nor the extensors predominating, but the one set 

 of muscles equalising the action of the opposite set. 



When a certain group of muscles is found to predominate in its 

 action in one root it, as a rule, predominates in that root, e.g., I have 

 not met with an instance in which the flexors of the digits did not 

 predominate over the extensors in their action when the 6th lumbar 

 nerve root was stimulated. 



If the muscles producing flexion of a certain joint predominate in 

 their action in one root, those producing extension predominate in 

 another. This does not, of course, apply only to when both the 

 opposing groups of muscles are represented in two nerve roots, but 

 also when they are represented in different nerve roots. We have 

 seen, for instance, that in the case of the ankle joint the muscles pro- 

 ducing dorsiflexion are represented in the 5th and 6th lumbar nerve 

 roots, while those producing extension at the same joint are repre- 

 sented in the 6th and 7th lumbar roots. This being the case, dorsi- 

 flexion results on excitation of the 5th lumbar root, and extension 

 re suits on excitation of the 7th. 



When two opposed movements are represented in three consecutive 

 nerve roots, the middle root of the series is that in which both move- 

 ments are represented, while the root above contains the one move- 

 ment and that below contains the other. Sometimes the two move- 

 * Cf. Patterson, Forgue, &c., loc. cit. 



