268 MESSRS. F. GOTCH AND V. HORSLEY 



In that communication we stated that the object of our work then ^^s to 

 endeavour to ascertain the character of the excitatory processes occurring in nerve 

 fibres, when, either directly (artificially) excited, or when in that state of 

 functional activity, which is due to the passage of impulses along them from the 

 central apparatus. The most important way in which such a method could be 

 applied was obviously one which would involve the investigation of the excitatory 

 changes occurring in the fibres of the spinal cord when the cortex cerebri is stimu- 

 lated. We must at once assume that the motor side of the central nervous system is 

 practically divisible into three elements : 1. Cortical centres. 2. Efferent (pyramidal 

 tract) fibres leading down through the internal capsule, corona radiata, and spinal 

 cord. 3. Bulbo-spinal centres contained in the medulla and the spinal cord, and 

 forming the well-known nuclei of the cranial and also of the spinal motor nerves. 



It had already been determined, both by direct observation and by the graphic 

 method (1) that certain areas of the cortex were connected with definite movements 

 of various parts of the body, and (2) that while the complete discharge of the cortical 

 apparatus was followed by a very definite and characteristic series of contractions of 

 the muscles in special relation with the particular point excited, the effectual removal 

 of the cortical central mechanisn and subsequent excitation of the white fibres passing 

 down through the internal capsule, &c., led to the production of only a portion of the 

 effect previously obtained from the uninjured brain. 



This method of observation in no wise showed what processes were actually 

 occurring in the spinal and other nerve fibres, and although the ablation of the 

 cortical centre, to a certain degree suggested the extent to which the cortex acted, 

 nevertheless, it did not afford an exact demonstration of the same. Moreover, the 

 data which the graphic method furnished were precluded, through their being 

 muscular records, from determining what share, if any, the lower bulbo-spinal central 

 nerve cells took, either in the production of the characteristic sequence of contractions 

 or in the modification, whether in quality or in force of the descending nerve impulses 

 during their transit. 



It seemed to us that the only way to approach this subject would be to get, as it 

 were, between the cortex and the bulbo-spinal system of centres. This would be 

 accomplished if some means were devised of ascertaining the character of the 

 excitatory processes occurring in the spinal fibres of the pyramidal tract, when upon 

 excitation of the cortex, nervous impulses were discharged from the cortical cells and 

 travelled down the cord. 



The question as to the extent to which it is possible to obtain physical evidence of 

 the actual presence in nerve fibres of excitatory processes and thus to arrive at 

 reliable data for the comparison of their amounts is one which, up to the present, has 

 been answered only indirectly, and that in two ways, firstly, by the extension of 

 HELMHOLTZ'S classical experiment of determining the rate of transmission, and 

 secondly, by observing those variations of electrical states in nerve fibres which 



