782 THE NERVOUS SYSTEM 



nuclei constitute the second links in the neurone chains conveying sensory impulses from the 

 periphery to the cerebral cortex. 



The descending or caudal branches of the dorsal root axones are concerned 

 wholly with the neurones of the spinal cord. They descend varying distances, 

 some of them as much as four segments of the cord, and give off numerous col- 

 laterals on their way to the cells of the grey column. Those terminating about 

 cell-bodies of the ventral horn which give rise to the ventral or motor root-fibres, 

 are responsible for certain of the so-called 'reflex activities' and thus contribute 

 to the simplest of the reflex arcs. In descending they serve to associate different 

 levels of the grey substance of the cord with impulses entering by way of a single 

 dorsal root. Some of their collaterals cross the mid-line in the posterior white 

 commissure, and thus become connected with neurones of the opposite side. The 

 caudal branches of longer course are scattered throughout the ventral portion of 

 the fasciculus cuneatus (middle root zone), and the longest show a tendency to 

 collect along the border-line between the fasciculus cuneatus and the fasciculus 

 gracilis, and thus contribute largely to the comma-shaped fasciculus. Also some 

 of the longest of them in the lower levels course in the oval bundle or septo- 

 marginal root zone. 



The ascending branches of the dorsal root axones also give off collaterals to the grey sub- 

 stance of the cord, thus extending the area of distribution of a given dorsal nerve-root to levels of 

 the cord above the region at which the root enters. 



The greater number of the terminations of dorsal root axones within the spinal 

 cord are concerned first with neurones other than those contributing ventral root- 

 fibres. The greater mass of the neurones concerned are those of the Golgi type II 

 and those contributing the fasciculi proprii or ground bundles of the spinal cord, 

 or the second variety of axones composing the posterior funiculus. The latter 

 fasciculi arise from the smaller cells of the grey column. 



These axones pass from the grey substance to enter the surrounding white substance, bifurcate 

 into ascending and descending branches, which in their turn give off numerous collaterals to the 

 cells of the grey substance of the levels through which they pass. The cell-bodies giving origin 

 to such axones are so numerous that the entire column of grey substance is surrounded by a 

 continuous felt-work of axones of this variety. 



The dorsal fasciculus proprius (anterior root zone of posterior column) arises chiefly from 

 cells situated in the dorsal horn (stratum zonale). Coincident with the ingrowth and arrange- 

 ment of the fasciculi gracilis and cuneatus many fibres of the dorsal fasciculus proprius go to form 

 both the oval bundle and the comma-shaped fasciculus. Thus these two bundles are mixed, 

 being fasciculi proprii which contain caudal branches of dorsal root axones. The association 

 fibres in the oval bundle are the longest of any belonging to the dorsal fasciculus proprius. The 

 cephalic and caudal branches combined of some are said to extend more than half the length of 

 the cord and it has been claimed that some even associate the cervical region with the conus 

 meduUaris. Based upon this claim, Obersteiner has called the oval bundle, the "dorso-medial 

 sacral field" and Edinger has referred to the most dorsal part of it as the "tractus cervico-lum- 

 balis dorsalis." The 'median triangle' is formed by the continuation of the dorsal fasciculi proprii 

 with the oval or septo-marginal fasciculus. Some of the axones of the dorsal fasciculus proprius 

 cross the midline to distribute impulses to the neurones of the opposite side. These commissural 

 axones, together with certain collaterals of the dorsal root axones, which cross the mid-line out- 

 side the dorsal white commissure, compose the so-called cornu-commissural tract at the base of 

 the posterior septum. 



The lateral funiculus or column [funiculus lateralis]. — Not all the axones of 

 the posterior or dorsal nerve-roots extend to the encephalon. Estimation shows 

 that the sum of all the dorsal roots is greatly in excess of the sum contained in the 

 fasciculi cuneatus and gracilis just before these enter their nuclei of termination. 

 Therefore many of the ascending dorsal root axones are concerned with spinal- 

 cord relations wholly. 



The marginal zone of Lissauer, situated along the lateral margin of the postero-lateral sulcus, 

 is composed largely of dorsal root axones. Many of these finally work across the line of the 

 sulcus into the posterior funiculus. Many of the dorsal root-fibres which do not reach the brain 

 occur in Jji.ssaiier's zone. Many others of course occur througho\it the posterior column. 

 Lissauer's zone also contains some fibres arising from the small cells of the dorsal horn, and to 

 this extent corresj)on(ls to a fasciculus proprius. Ranson has foimd that large numbers of the 

 non-medullatcd dorsal root axones wliich enter the cord are contributed to Lissauer's zone. 



The lateral fasciculus proprius (lateral ground bundle, lateral limiting layer) 

 is situated in the lateral concavity of tiie grey column and is continuous with the 

 other fasciculi proprii botli dorsal and ventral. Beyond that it probably contains 



