ON THE MAMMALIAN NERVOUS SYSTEM. 477 



with the fibres of the posterior roots on the same side than they have with those on 

 the opposite side, these last being scarcely represented. 



The most striking differences between all the connections of the posterior roots with 

 the cord, as dependent upon whether impulses are made to ascend or descend 

 them, are 



i. For ascending impulses the lateral column of the same side affords indirect 

 channels of greater efficiency than the crossed path in the opposite posterior column, 

 or, a fortiori, the opposite lateral column. 



ii. For impulses descending along afferent paths the lateral column affords indirect 

 channels of less efficiency than the crossed path in the opposite posterior column. 



iii. The ease with which the ascending impulses evoked by "minimal" excitation 

 cross from the posterior root into the posterior column of the opposite side is con- 

 siderable as compared with that which characterises the passage backwards from a 

 posterior column into the opposite nerve roots when descending impulses are evoked 

 by such "minimal" excitation. 



These differences tend to show that although there ^is no absolute block to the 

 backward passage of impulses descending afferent channels into afferent fibres of the 

 nerve, such as exists in the anterior roots to the passage upwards of ascending impulses 

 in efferent channels, yet there is a physiological difference between the facilities 

 which afferent indirect paths offer to the passage of impulses, this passage being 

 much easier when it occurs in the normal ascending direction. 



We now consider ourselves warranted in concluding that, as far as the afferent 

 tracts are concerned, the deductions of WOROSCHILOFF, MIESCHER, &c., referred to 

 on p. 420, are founded on experimental results which, from their nature, readily 

 admitted of misinterpretation, and that both physiological experiment and anatomical 

 investigation point with great distinctness to what is the true afferent path. Our 

 observations confirm the views as to the physiological properties of the posterior 

 columns set forth in 1847 by LONGET and reasserted by SCHIFF, with this modifica- 

 tion, that although the posterior columns form the main path of connection between 

 higher portions of the cord and the posterior roots of the nerves, yet there is a path 

 in the lateral column which is strictly confined to the side of the entering roots. 



It may be urged that there are pathological cases (BHOWN-SEQUARD) which 

 undoubtedly exist, of motor paralysis on one side and hemiansesthesia on the other, 

 following a local lesion of the cord. In reply, we can only state that we believe much 

 more definite evidence than that afforded by the existing clinical data must be forth- 

 coming in order to shake the solidity of the foundations for our propositions, a solidity 

 derived from the welding together of all the foregoing quantitative results of exact 

 experiments. We therefore are compelled to regard the above clinical experiences as 

 either in no way indicative of the normal relations of the cord to the nerves, or as 

 capable of explanation on the supposition that the lesion has affected at the same time 



