678 NERVOUS SYSTEM. 



seemed to show that the posterior columns of the cord were the conductors of sensory 

 impressions to the brain, and that the antero-lateral columns transmitted the motor stim- 

 ulus. These were made by applying a stimulus directly to the cord itself. Longet dis- 

 credited observations made by dividing different portions of the cord, for the reason that 

 he supposed that the mere operation of exposing the cord and of removing the dura 

 mater was followed by a depression of the nervous action sufficient to render the evidences 

 of sensibility in the lower extremities scarcely appreciable. The conclusions drawn from 

 these experiments were at first accepted by nearly all physiological writers, and it was 

 generally admitted that the transmission of sensory impressions was effected solely by 

 the posterior columns. It was found that the gray matter of the cord was both insen- 

 sible and inexcitable, and the conduction was supposed to take place exclusively through 

 the white substance. The views of Longet, however, were in direct opposition to those 

 of Bellingeri, who claimed, in 1823, to have demonstrated by experiment, that sensory 

 impressions were conveyed to the brain exclusively by the gray substance of the cord, 

 and that sensibility persisted in the lower extremities after complete section of the pos- 

 terior white columns. 



At the time the above-mentioned experiments were made, our knowledge of the prop- 

 erties of the cord was very incomplete, and it was difficult to understand how any of its 

 fibres could conduct sensory impressions and yet be insensible to direct stimulation ; but 

 now we know that the gray matter does act as a conductor, and yet it is certainly insen- 

 sible. The simple questions now to be determined are the following : 



1. Does or does not the white substance of the posterior columns of the cord conduct 

 sensory impressions to the brain ? 



2. Does the entire gray substance of the cord act as a conductor of sensation ? 



3. Do both the gray matter of the cord and the white substance of the posterior col- 

 umns act as conductors, or does either one act to the exclusion of the other? 



These questions may now be considered as definitively answered by the most positive 

 and unmistakable results of experiments upon living animals, which, while they render 

 the precise function of the white substance of the posterior columns to a certain extent 

 a matter of conjecture, leave no doubt with regard to the parts of the cord which act as 

 conductors of sensory impressions. 



The experimental answer to the first question is capable of but one construction. If 

 the white substance of both posterior columns be divided, the sensibility of the posterior 

 extremities is not diminished, at least as far as can be shown by experiments upon ani- 

 mals, in which these points are always difficult of determination. On the other hand, if 

 every portion of the cord be divided except the posterior white columns, sensibility is 

 completely lost in the parts below the section. The accuracy of these results cannot be 

 called in question, especially when controlled by experiments showing the conducting- 

 properties of the gray substance of the cord ; and they show that, whatever may be the 

 functions of the posterior white columns, they do not serve as conductors of sensory 

 impressions. 



The second question admits of an equally positive answer from the results of experi- 

 mental inquiry. If the entire substance of the cord, except the posterior columns of 

 white matter, be divided transversely, as we have just seen, sensibility is abolished in all 

 parts below the section ; but, as we have stated in treating of the transmission of motor 

 stimulus by the cord, voluntary motion is also destroyed. Experiments show, farther- 

 more, that sensory impressions are conveyed exclusively by the gray substance. "If the 

 anterior, the lateral, and the posterior columns of the spinal cord are divided transversely, 

 at the dorsal region, one set at one place, another at a distance of one or two inches, and 

 the third also at the same distance from the second, so that the only channel of commu- 

 nication between the posterior limbs and the sensorium is the gray matter, of which, 

 however, several parts have, unavoidably, been divided (such as the anterior and the 

 posterior gray cornua, and also more or less of the central gray matter), we find that the 



