FUNCTIONS OF THE SPINAL CORD AS A CONDUCTOR. 681 



injuring the white substance, and, as we are fully acquainted with the motor properties 

 of the cord, we are prepared to comprehend the effects upon conduction of sensory im- 

 pressions which follow division of one or the other lateral half. In our detail of experi- 

 ments, we shall not consider the phenomena of hyperassthesia, but confine ourselves to 

 the loss or diminution of sensibility. 



Brown-Se'quard was the first to demonstrate decussation of the sensory conductors in 

 the cord itself; and, although his experiments upon this subject are almost innumerable, 

 and his writings, scattered, voluminous, and sometimes not free from the obscurity due 

 to unnecessary refinement and elaborateness of detail, the main facts can be expressed in 

 a very few words; and he may justly be said to have created the physiology of the sen- 

 sory conductors. 



Brown-Sequard repeated the experiments of Galen and of Fodera, dividing the cord 

 longitudinally in the median line, producing complete paralysis of sensation upon both 

 sides in all the parts below the section. By this operation, if the section had been made 

 accurately in the median line, the only fibres that could be divided were those passing 

 from one side of the cord to the other. 



The second experimental proof of the decussation of sensory fibres consists in trans- 

 verse section of one or the other of the lateral halves of the cord. If one lateral half of 

 the cord be divided, sensibility is abolished in the parts below the section, upon the oppo- 

 site side of the body. In an article published in 1858, Brown-Sequard details very suc- 

 cinctly an experiment showing this fact, though his first experiments were made in 1849. 

 He denuded the cord in the lumbar region in a vigorous dog, and made sections upon one 

 side, progressively deeper and deeper, from without inward. When the section included 

 about one-third of the lateral half, the sensibility seemed slightly augmented upon the 

 opposite side. This section involved only a part of the lateral w T hite column and a small 

 portion of the anterior cornu of gray matter. When the section was extended so as to 

 involve about two-thirds of the lateral half, the sensibility was notably diminished upon 

 the opposite side. When the section extended to the median line, the sensibility was 

 very much diminished ; and, when it extended just beyond the median line, it was entirely 

 abolished upon the opposite side. These observations, and others of the same nature, 

 show conclusively that, in the animals experimented upon at least, there is a decussation 

 of the greatest part of the sensory conductors in the cord itself. 



The course of the fibres in their decussation is indicated by farther experiments, which 

 show that the sensitive fibres from ,the posterior roots of the nerves " pass along the pos- 

 terior columns only a little way, and leave them to enter the central gray matter." It is 

 undoubtedly in this gray substance that they pass from one side to the other, probably 

 through the cell-prolongations. The fact that the fibres pass in the cord a short distance 

 before they decussate, and that they pass downward as well as upward, is well shown by 

 the following experiment : 



" If we divide transversely a lateral half of the spinal cord in two places, so as to have 

 three pairs of nerves between the two sections, we find that the middle pair has almost 

 the same degree of sensibility as if nothing had been done to the spinal cord, while the 

 two other pairs have a diminished sensibility, the upper one particularly in its upper 

 roots, and the lower one in its lower roots ; which facts seem to show that the ascending 

 fibres of the upper pair, and the descending fibres of the lower one, have been divided 

 before they had made their decussation. 



" If there is only one pair of nerves between two sections, its sensibility is almost 

 entirely lost, as then the transversal fibres are almost alone uninjured (most of the ascend- 

 ing and descending being divided), which fibres are employed for reflex action, and hardly 

 for the transmission of sensitive impressions." (Brown-Sequard.) 



The experimental facts just cited conclusively show decussation of sensory conductors 

 in the cord in the animals operated upon ; and this has been sufficiently confirmed by 

 other experimenters to render the fact certain. It is possible that the crossed action may 



