776 DR FRASER ON THE PHYSIOLOGICAL ACTION 



From the nearly complete absence of blood-vessels in the trunks of the sciatic 

 nerves, it is possible to localise the effects of a poison to any portion of the trunk. 

 A curious result was produced by the topical application of Calabar bean. It has 

 been shown that this poison does not appear to paralyse the afferent nerve fibres 

 when acting through the blood ; or, at least, that under its influence the function 

 of the motor nerves is indubitably very much sooner destroyed than that of the 

 sensory. When, however, the poison is applied to a mixed nerve-trunk, the order 

 wherein these effects are produced is reversed, the afferent nerves being paralysed 

 a few minutes before the efferent. This may be shown with great distinctness if 

 strychnia be given after the local action has continued for some time. 



Experiment XCIV. 



The spinal cord of a frog was divided at the occiput, the sciatic nerves were exposed, and a 

 piece of gutta-percha parchment was placed under each nerve, so as to isolate it completely. A 

 small pad of cotton wadding steeped in water, was applied to the right nerve, while a similar 

 pad steeped in a concentrated mixture of extract of physostigma and water, was placed on the left; 

 care being taken to prevent the diffusion of any of the extract beyond the parchment. At this 

 time, a slight stimulation of either nerve, below or above the pads, caused contraction of the limb 

 and general reflex movements. 



The pads were kept moist by an occasional drop of water on the right, and one of watery 

 extract of physostigma on the left ; and they were retained in their positions for an hour and 

 forty minutes. They were then removed, and the left leg was carefully washed with dis- 

 tilled water, so as to remove effectually any extract that might have been adhering to the nerve. 

 They were both tested with galvanism, when movements of the left leg followed the application 

 of the poles to any portion of the exposed left sciatic ; but no reflected contraction occurred 

 when the nerve was stimulated below the position that had been occupied by the pad. No change 

 had occurred either in the afferent or the efferent conductivity of any portion of the right nerve. 



A drop of solution of strychnia was now applied to the wound that had been made in the 

 neck by dividing the cord. Seven minutes after this, very weak galvanism of any portion of the 

 right sciatic nerve caused a spasmodic shock of all the body. The same current produced a like 

 effect when it was applied to the left nerve above the part that had been occupied by the poisoned 

 pad ; hut when it was applied to the poisoned part, or lower down, the muscles below the point 

 stimulated alone contracted, no reflected movements being caused. The same effects were 

 repeatedly observed for other six minutes, before the lapse of which time the action of the 

 strychnia had manifested itself more violently, and rendered the above peculiarities more 

 distinct and exaggerated. 



This experiment has been several times repeated, and has always yielded 

 similar results. 



If the poisoned pad be permitted to remain in contact with the nerve for a 

 few minutes after the paralysis of its afferent fibres has been caused, the motor 

 fibres also will have their conductivity destroyed. 



Experiment XCV. 



The sciatics of a frog were exposed, and were treated exactly as in the previous experiment. 

 The afferent conductivity of that portion of nerve to which physostigma had been applied was 

 lost in two hours, but the motor conductivity was yet retained. The application was continued, with 

 the result that the conductivity of the motor fibres was destroyed within other fifteen minutes. 



