630 FUNCTIONS OP THE CEREBRO - SPINAL NERVOUS SYSTEM. 



mal, that the hind limb of the side opposite to that on which the section is 

 made, becomes progressively less and less sensitive, till at last when the sec- 

 tion has passed a little beyond the middle line it is entirely lost. But if 

 the nerve-fibres ministering to common sensation on both sides decussate 

 in the spinal cord, it must follow, that on making a longitudinal section 

 from above downwards, along the whole length of the cord, dividing it into 

 two lateral halves, sensibility of both sides of the body should be totally 

 extinguished, for such a section would obviously divide all the sensory 

 nerves, and, according to M. Browu-Sequard, this is actually the result 

 obtained. Other observers, however, as Ore, 1 Louget, and Schiff, though 

 admitting there is great diminution of sensibility after a longitudinal divi- 

 sion of the cord, maintain that distinct evidence of the conduction of sensory 

 impressions may still be obtained ; and it seems probable from their ex- 

 periments, that some few fibres may ascend in the gray substance or in the 

 posterior white columns of their own side. Perhaps the discrepancies in the 

 results of the operation in the hands of different experimenters are due to 

 different animals having been used ; for M. Brown-Sequard has himself ob- 

 served, that the decussation of the sensory fibres is neither so complete or 

 immediate in Reptiles and Birds as in Mammals; and he has further noticed, 

 that it does not seem to be so perfect in the lumbar as in the dorsal region. 2 

 The important experiments made by M. Browu-Sequard show clearly that 

 whilst the sensory fibres of the posterior roots in part run at once to the op- 

 posite side of the gray substance, some of them ascend and others descend 

 for a short distance in the gray substance before decussating. Thus, if in one 

 animal the posterior columns be divided above the origin of a particular 

 pair of nerves, and the posterior columns be then dissected back for a short 

 distance (Fig. 236), whilst in a second animal a section be made immedi- 

 ately below the origin of the same pair of nerves, and the same dissection be 

 made (Fig. 237), in the former case the flap will only communicate with the 



FIG. 236. 



FIG. 237. 



7 



gray substance by its lower extremity, and in the latter by its upper ex- 

 tremity. In both instances the animal will continue to give indications 

 when the nerve (Ti] is pinched, that sensory impressions are still conveyed 

 to the sensorium ; thus showing that some of the fibres descend whilst others 

 ascend in the posterior columns, for some distance before entering the gray 

 substance, following in the two cases the course marked in the figures by the 

 lines u, a, u, b. By making sections beginning at K, two centimetres from 

 the point to which the posterior column had been reflected, where sensation 

 was found to be still acute, and gradually approximating the attached por- 

 tion of the posterior column till sensation was found to be abolished, Brown- 

 Sequanl aseertainod that in the dog the posterior roots after their entry into 

 the posterior columns spread upwards and downwards for a distance of 4 or 

 5 centimetres, or from an inch and a half to one inch and three-quarters be- 

 fore entering the gray substance. The sensory fibres which enter motor 

 ganglionic cells of the anterior cornua, and are subservient to reflex acts, 



CompU-s Rundus, t. 38, 1854. 



Op. cit., p. 37. 



