ON THE MAMMALIAN NERVOUS SYSTEM. 290 



be seen in the chapters which deal with the different groups of results, how the effects 

 obtained in slight anaesthesia differ from those obtained with similar stimulation in 

 profound narcosis. It was, therefore, extremely important to keep the condition 

 of anaesthesia as far as possible even, and to note as a guide to that condition 

 the presence or absence of muscular movements, when any part of the central nervous 

 system was stimulated. 



In one respect, the degree of etherisation was most useful, since it served as 

 a valuable control against error due to simple electrical escape from the exciting 

 into the galvanometric circuit. It is evident, if on pushing the degree of narcosis 

 the electrical changes in the observed tissue are very much diminished or even 

 abolished, whilst the excitation by electrical current remains unaltered, that no 

 physical conduction of derivation currents from the exciting into the galvanometric 

 circuit can be present. 



(c.) Collapse. Since the experiments are often necessarily spread over a consider- 

 able period of time, the gradual collapse of the animal is not infrequent. Its onset is 

 readily distinguished, and can be allowed for by diminishing the amount of etherisation 

 (see p. 282), and by increasing the intensity of the stimulus used. 



(d.) Hyperexcitability. In connection with this part of the subject must be noted 

 the variations in excitability of parts due to section. 



As regards the cortex it has occasionally happened that it has been slightly punctured 

 by an electrode point, &c. Whenever this happened the effect was to heighten the 

 excitability of the immediate neighbourhood, and to produce thereby a very marked 

 epileptogenous change. A more extensive injury has the further effect of abrogating 

 not only this increase of excitability, but the normal degree as well. 



As is well known, the excitability of nerve fibres is raised by their section, and 

 from a rapidly attained acme the excitability steadily diminishes, passing the normal 

 point and becoming subnormal. FRANCOIS FRANCK has shown the same to be true 

 for the reflex centres in the spinal cord, when division of the latter has been carefully 

 made high up and without notable haemorrhage. We have always borne this in 

 mind, and have discounted the rise and fall of excitability due to the section in each 

 observation. 



(4.) Localisation of the Excitation, &c. 



Many precautions had to be observed in carrying out the stimulation of the cortex, 

 spinal cord, and nerves respectively, especially when the excitation of these structures 

 was electrical. 



A. Cortex. As previously stated, it was, of course, found imperatively necessary 

 to keep the surface of the cortex both warm and moist, this being especially achieved 

 by replacing the skin and by irrigation with warm saline solution. To avoid short 

 circuiting and consequent irregular intensity of the exciting stimulus, the surface of 

 the brain was always carefully dried with soft sponge or amadou immediately before 



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