356 ANATOMY OF THE CENTEAL NEETOUS SYSTEAI. 



be regarded as sensory is doubtful. To me it seems most probable that the 

 sensory fibers are here spread out over the entire cross-section of this column. 

 Still, miich is in favor of the view that the anterior radicular zone contains 

 many of these secondary sensory fibers. 



We have learned of two kinds, then, of extension of the posterior root- 

 fibers: one direct in the posterior columns, and another indirect, which 

 only by connecting with a secondary decussating tract takes a cerebral 

 direction. We will see later that the u.ncrossed fibers encounter higher up 

 in the medulla cells, there located, and then cross over; so that the whole 

 tract finally decussates. 



It were scarcely possible to draw these conclusions regarding the pos- 

 terior root-fibers were it not true that in the lower vertebrates, in the spinal 

 cord, the relations are very simple, — one might even say schematic. After 

 it had been demonstrated with them that the majority of their posterior 

 root-fibers ran into a central gray deposit of fibers and cells, and from there 

 new tracts, after crossing, extended on toward the brain, it was but a step to 

 look for corresponding conditions with man and mammals. The discovery 

 by Eamon y Cajal that the posterior root-fibers split up around the cells of 

 the spinal gray matter, and that from these cells there came out a tract 

 which crossed over in the anterior commissure, gave to the supposition some- 

 thing of definiteness. 



Witli this new achievement of knowledge coincide well the results of experi- 

 mental and clinical pathology. If, for instance, a spinal cord were cut through its 

 lateral half, underneath the point of section the tactile sense would be lost, — not 

 on the corresponding side, but on the opposite one. This datum could not be 

 reconciled with what was known of the crossed extensions of the posterior root- 

 fibers in the posterior columns. It was, however, soon cleared up, when we learned 

 that u, considerable portion of such a root, soon after its entrance into the cord, 

 was continued by a secondary tract over to the other side. 



One must not suppose that all the impulses reacliing the spinal cord by the 

 sensory roots are identical \\ith what is ordinarily termed "sensation."' In order 

 that an impression be perceived, it is not sufficient that it be conducted to the 

 spinal cord, but it must be farther carried up, from the place where the peripheral 

 path ends, to the cerebral cortex. There is, ho\Never, no doubt at all that these 

 higher connections are few in number, and that, contrasted with the multitude of 

 fibers in the posterior roots, the number of such central connections is quite small. 

 This alone makes the conclusion possible that there are, indeed, many sensory im- 

 pressions ^vhich arrive at the spinal cord, but that we are aware of but few of them 

 at the time. All the viscera of the body, as the silver staining-method has distinctly 

 shown, are traversed by an altogether unexpectedly large number of nerves, and 

 their arrangement and course, their relations to blood-vessels and glands, and to 

 muscle-fibers, bones, and enamel, make it more than probable that there is, in this 

 connection, a large system \\'hich serves essentially to regulate impressions and reflex 

 action (Exncr). This is often overlooked in analyzing the results of section of cer- 

 tain eohuims. Until lately only the very coarsest qualities of sensation have been 

 tested. And even now, when we kno-w so much better tlian formerly the anatomic 



