294 MESSRS. F. GOTCH AND V. HORSLEY 



SECTION 4. THE GENERAL PROCEDURE AND THE PRECAUTIONS USED BY THE 



AUTHORS. 



A . The Method of Experiment. 



In any individual experiment in which excitatory electrical changes were observed 

 the procedure we adopted was as follows : 



The animal having been anaesthetised and immobilised, the structure to be 

 investigated was exposed and prepared in the manner indicated. It was always, 

 whether spinal cord or nerve, divided, and the soft cables of the non-polarisable 

 electrodes were then tied, one round the structure close to the point of division, the 

 other about 1 centim. away round the surface. The ligature upon the cut end 

 enabled this portion of the tissue to be suspended with its cables in air, without 

 dragging on the remaining portion of the tissue which remained in situ. The cables 

 were then connected with the non-polarisable electrodes. (See Plate 29.) 



The electrical difference between the two contacts was first observed and balanced 

 by the introduction by means of the compensator of a suitable difference opposite in 

 sign to the tissue difference. The general characters of the latter and the amount of 

 the balancing difference were then noted, both the galvanometer and electrometer 

 being used for this purpose. 



One of us devoted himself to the observation and record of the electrical changes ; 

 the other to the maintenance of the animal in a uniform condition of anaesthesia, to 

 the prevention of the drying, &c., of the investigated tissues, to the observation of 

 the muscular contractions, and to the excitation. 



The tissue to be excited, which had been exposed with that under investigation, 

 was then finally prepared, and the points of the electrodes were brought into contact 

 with it under all the precautions to be immediately described. 



The electrodes were kept short circuited as already stated by means of a key under 

 the control of the observer at the galvanometer, &c. When everything appeared 

 favourable and the galvanometer needle had been brought to its zero position by 

 suitable compensation the revolving exciting key was set going and the control key 

 opened for one revolution of the revolver ; an excitation of definite duration and 

 intensity was thus applied to the excited tract, whether cortex, corona radiata, spinal 

 cord, or nerve. The extent of any deflection of the galvanometer needle or the move- 

 ment of the meniscus of the mercurial column was noted, and special attention paid 

 to its character. The extent recorded was always that between the previous resting 

 position and the point where the moving recorder, whether spot of light or image of 

 meniscus, stopped and commenced its return. If no return occurred but only a 

 deflection produced which continued to creep on then the observation was disregarded 

 as worthless. Such an effect was found to be generally associated with some move- 

 ment of the animal which caused a slight displacement of the electrode contact. 



