ON THE MAMMALIAN NERVOUS SYSTEM. 439 



D. Previous Section of Posterior Columns some time Previous to Observation. 



In one Cat (251) we divided the posterior columns at the level of the 10th dorsal 

 vertebra four weeks before the experiments. When the animal (Cat) was examined 

 it was noticed that the water test showed diminished sensibility in both hind feet, 

 since the animal could be placed with its hind paws in water without exhibiting any 

 movements of withdrawal. There were, however, vague symptoms that the two sides 

 were not equal in the degree of sensibility they still possessed. This inequality 

 showed itself in the knee jerks, the left being much feebler than the right. On 

 placing the animal under ether, however, the left knee jerk was found to be then very 

 much exaggerated, and the left hind limb was easily sent into a condition of clonic 

 spasm. 



The animal was first experimented upon by exposing and exciting the cord, and 

 observing the changes in the nerves ; the result of this experiment will be given in 

 Chapter X. It was then used for the present series of experiments, the cord being 

 originally divided at the 5th dorsal vertebra, and the upper end of the lower fragment 

 prepared for galvanometric observation' at about the 7th dorsal vertebra. 



The left sciatic, when excited, with a minimal and a stronger stimulation, evoked 

 effects of 10 and 20 ; the right, when similarly excited, evoked cord effects of 

 8 and 70. 



The cord, after the death of the animal, was examined histologically, when it was 

 found that the section had not been complete on the right side, a good many fibres 

 having been left intact. On the left side it was complete, and at the level of the 7th 

 dorsal, on microscopical examination, a sickle-shaped patch of degeneration could be 

 seen in the postero-external column on each side, that on the left being very well 

 marked, whilst that on the right was small. The same difference between the 

 degeneration on the two sides could be seen at the level of the 4th cervical, where 

 the degenerated area occupied, on the left side, a conspicuous portion in the middle 

 third of the posterior median column. The discrepancy between the effects obtained 

 between the two nerves is thus cleared up by the minute examination of the extent 

 of the lesion. It furnishes striking evidence of the accuracy with which the electrical 

 method can gauge the integrity of through tracts of fibres, but brings out a disadvan- 

 tage, namely, that it involves the death of the animal ; hence, when lesions, &c., 

 are made at the time of the experiment, it is impossible to be perfectly certain of 

 their extent, since the degeneration method, which alone would give absolute indica- 

 tions, cannot then be used. 



SECTION 8. INFLUENCE OF SECTION OF THE LATERAL COLUMN. 



Of the remaining columns in the cord, the lateral and anterior, it is only the former 

 which appear to bear any relation to the production of the electrical effect in the 



