1881.] 



The Brachial and Lumbo- Sacral Plexuses. 



15 



seems, in fact, warrantable to make the constitution of the lumbar and 

 sacral plexuses a means of determining to which group the variable 

 dorsal, lumbar, and sacral vertebrae in monkeys should be referred. 



The mode of operation carried out by Dr. Yeo was as follows : — The 

 animals were deeply narcotised with chloroform, the vertebrae over the 

 cervical or lumbar nerves exposed, the arches removed so as to expose 

 the cord. The dura mater was opened, and in the case of the cervical 

 and dorsal nerves the posterior roots were cut in order to avoid all 

 reflex movements on stimulation. In the case of the roots forming 

 the cauda equina this was not necessary, as the anterior and posterior 

 roots run a long separate course, and can easily be isolated. The 

 motor roots were stimulated with closely approximated needle elec- 

 trodes by means of the induced current of Du Bois Raymond's secon- 

 dary coil. A minimal current, barely perceptible on the tongue, and 

 just sufficient to produce distinct action, was employed, the distance 

 of the secondary coil varying from 20 centims. or less, according to 

 the degree of excitability of the nerves, which is liable to considerable 

 variation at various stages. Every precaution was taken to insulate 

 the roots, and unavoidable diffusion was more or less eliminated by 

 frequent repetition and uniformity in the results. Occasionally, in the 

 case of the brachial plexus, where the clear exposure of the roots is 

 somewhat difficult, we used as one pole a flat electrode over the sacrum 

 as a neutral point, and stimulated the root by hooking it up on a 

 curved needle, which formed the other pole of the circuit. The roots 

 were stimulated either inside or outside the sheath, according to where 

 they were most conveniently reached. 



In observing the effects of stimulation, we directed our attention 

 more especially to the resultant muscular combination rather than to 

 the mere number of the muscles thrown into action. It will be seen 

 that this is of no little importance, as the actions excited are all com- 

 plex co-ordinated movements of great; significance. It is very difficult, 

 where so many events are occurring simultaneously, to analyse each 

 muscular combination into the individual factors at work; and there- 

 fore the muscles which we state to be in action are those which we 

 have collected from various observations, the muscles being felt or 

 partially exposed where it was difficult to be sure of their co-operation 

 otherwise. But we would not exclude, except when so stated, the co- 

 operation of other muscles which were necessarily removed from actual 

 observation, except by dissection. 



But we believe that these may be fairly determined by anatomical 

 considerations and by ascertained principles respecting the physiology 

 of movements applied to the individual actions described. 



We have experimented seven times on the brachial plexus, once un- 

 successfully ; and six times on the lumbo- sacral plexus, twice with 

 only partial success on two roots. 



