ON THE MAMMALIAN NERVOUS SYSTEM. 491 



discharges, we will proceed to show what fresh light the galvanometric method is able 

 to throw on the character of the issuing impulses. 



SECTION 3. CHARACTER OF NERVE IMPULSES DISCHARGED BY THE 



SPINAL CENTRES. 



It seemed to us that an estimate of the amount of the impulses which pass along the 

 nerves as the discharge of a reflex centre, might be obtained by comparing the amount 

 of the electrical change in a peripheral nerve when (a) the centre is reflexly aroused, 

 and (b) the nerve itself is subjected to a stimulus of similar duration and strength. 

 Such an estimate can only give approximate results, but the difference revealed 

 in the two cases is so striking, and the desired comparison of so much importance 

 in attempting to analyse the components of a reflex, that we have devoted some 

 time to securing it. We have done this for both a "simple " reflex, i.e., that obtained 

 from the same side as that of stimulation, and we have also measured the effect when 

 it is the result of a " crossed " reflex. 



(1.) The Excitatory Electromotive Change Produced in a Mixed Nerve when a 



Spinal Centre is Excited Reflexly. 



(a.) Simple Reflex. To obtain the electrical effect produced by a simple reflex 

 discharge into the peripheral nerves, we divided the spinal cord in the dorsal region, 

 then prepared and divided the sciatic nerve and connected its central end with the 

 galvanometer. The cauda equina was exposed to a limited extent, and a posterior 

 root of the same side selected for excitation ; this was divided near the ganglion and 

 its central end stimulated. (See fig. 20.) 



The excitation was performed for a known time, as in all our experiments, and the 

 galvanometric effect obtained in the nerve noted. We then applied the same, or a 

 weaker stimulus of the same duration to the observed nerve. The results obtained 

 form a remarkable contrast. The strength of the stimulus is so greatly altered 

 in its effect by the condition of the animal, rest, depth of narcosis, &c., that what 

 is minimal in one case, is quite adequate, or maximal in another. 



The readings of the deflections indicating the reflex electrical effects in the nerve 

 were : 



Highest readings .... 72 (iO 55 (three highest taken). 



Average 26 



Lowest readings .... 7 3 nil (three lowest taken). 



Out of the whole number of observations, 9 per cent, gave no result. Finally, the 

 stimulus employed on the average amounted to 2500 of the inductorium scale. 



Taking the average result, namely, 26 of the galvanometer scale, it is at once 



3 R 2 



