ON THE MAMMALIAN NERVOUS SYSTEM. 451 



now a larger and now a smaller effect, dependent upon marked lessening and deepening 

 of the narcosis. The importance of bearing this in mind is sufficiently obvious ; it is, 

 however, capable of control, since the character of the movements awakened by the 

 stimulus furnishes a fair index of the condition of anaesthesia, and was in all cases 

 closely observed. 



Finally, the influence of temperature is one to which attention has already been 

 directed in Chapter III., and although the necessity of keeping the exposed cord 

 covered with warm sponges, except during the actual stimulation, has been already 

 dwelt upon, it may be insisted on again at this juncture. 



Alterations in the duration and intensity of the stimulus modify the effect ; the 

 longer the duration and the greater the intensity, the more pronounced is the nerve 

 change. The modifications are so marked, that in analysing and comparing the 

 quantitative value of the nerve effects obtained by stimulation of different regions of 

 the cord, it will be necessary to divide the indicative galvanometric deflections into 

 two classes, as evoked by &quot; minimal &quot; and maximal stimuli respectively. It will be 

 understood, however, that a rigid separation is impossible, there being every grade 

 between an undoubtedly &quot; minimal&quot; effect with no accompanying reflex movements 

 and an undoubtedly maximal effect with vigorous movements. 



The experiments were made upon twenty-one Cats and six Monkeys, and may be 

 divided into four groups, the first three of which differ as regards the nerve fibres in 

 which the electrical change was observed, the fourth differing as regards the 

 condition of the cord, through the stimulation of which the nerve effect is evoked. 

 We have, therefore, to consider in succession : 



The electrical effect in the mixed sciatic nerve ; 



,, afferent nerve roots ; 



,, efferent nerve roots ; 



The modifications produced by intervening section of tracts in the cord. 



Each group will form the subject of a succeeding section. 



SECTION 3. THE ELECTRICAL EFFECTS IN THE SCIATIC NEKVE FOLLOWING 

 EXCITATION OF THE DIFFERENT COLUMNS OF THE CORD. 



The experiments upon this subject will be best displayed by first selecting and 

 describing the results of a particular experiment in the Cat and Monkey respectively, 

 and then giving a table which will show the averages of all the comparable 

 observations. 



The spinal cord was exposed and divided in an anaesthetised Cat (331), at a level 

 between the 10th and llth dorsal vertebrae; both the sciatic nerves were then care 

 fully exposed, ligatured in the lower part of the thigh, and divided on the peripheral 

 side of the ligature. Tliey were then freed from their attachments and raised in the 



3 M 2 



