CENTRAL NERVOUS SYSTEM. 227 



collaterals and the ascending branches of the afferent axones, which end 

 before they reach the nuclei of the dorsal funiculi. The cells in the dorsal 

 horns send their axones in large numbers across the cord to the lateral 

 columns of the opposite side, to reach the thalamus through the medial 

 lemniscus, and thence to the cortex. 



Of the many disputed points in this pathway, the most important relates 

 to the interruption of the axones of the lemniscus in the ventral portion of the 

 thalamus. The recent researches of Tschermak ' indicate that probably there 

 are two groups of neurones concerned, one of which sends its axones without 

 interruption to terminate in the cortex, while the axones from the other are 

 interrupted at the level of the thalamus. The latter group is the larger and 

 probably the more important for the general reactions of the central system. 



The pathways which are here sketched have been worked out mainly by 

 the study of degenerations, in large part resulting from experimental lesions. 



When the dorsal roots are crushed or sectioned between the spinal gan- 

 glion and the cord, the prolongation of the afferent fibre within the cord 

 degenerates throughout its entire extent. The degeneration extends in the 

 dorsal columns down the cord two or three centimeters from the level of the 

 section, and also up the cord as far as the nuclei of the dorsal columns located 

 at the commencement of the bulb. If the section is made near the caudal 

 end, the degeneration may in consequence run through the entire length of 

 the cord. Moreover, it occurs mainly on the side of the cord to which the 

 sectioned nerves belong. Take, for example, the area of degeneration caused 

 by the section in a dog of the dorsal roots on the left side between the sixth 

 lumbar and second sacral nerves. The degeneration in the lower lumbar 

 region is represented in Fig. 97, a, in the upper lumbar region in b, and in 

 the thoracic region in c and d. The section c passes through the cervical 

 enlargement. On passing cephalad the area of degeneration becomes smaller. 

 This is interpreted to mean that all along, between the caudal and cephalic 

 limits, fibres are given off from the main bundle to the intermediate levels 

 of the cord. Here is evidence of an arrangement that is always t<> be kept 

 in view. Though a number of fibres among those degenerating alter section 

 of the dorsal roots may run the longer course to the bulb, the larger portion 

 run a short or an intermediate course, and are, therefore, distributed at dif- 

 ferent points between the termini. Injury to the dorsal roots at different 

 levels shows, moreover, that the fibres from a given level which run the 

 length of the dorsal columns do not mingle indiscriminately with those from 

 other levels, but form a bundle; and as that bundle passes cephalad in the 

 cord, it tends to lie nearer the middle line. Hence in the upper cervical 

 cord, where the bundles from all levels are present, a cross-sect ion shows the 

 bundles which entered lowest down to be located nearest the dorsal surface 

 and the median septum. 



From these relations it is evident that comparatively few of the dorsal 

 root-fibres run the entire length of the dorsal funiculi, since the majority 



1 Tschermak: Archivfur Anatomii v/nd Physiologie, Anat. Abthl., 1898, S. 291-400. 



