THE CENTRAL NERVOUS SYSTEM. 



1027 



The Columns of the Cord. — Inspection of the surface and particularly of 

 cross-sections of the spinal cord (Fig. 885) shows tlie latter to be partially divided 

 into a symmetrical right and left half by a median cleft in front and a partition in the 

 mid-line behind. The cleft, the anterior median fissure (fissura mediana anterior) 

 extends the entire length of the cord, and is continued on the upper part of the 

 filum terminale. It is narrow, from 2-3.5 "''"''• ^^ depth, penetrating for less than 

 one-third of the ventro-dorsal diameter of the cord, and occupied by a process of 

 pia mater. Along its floor, which lies immediately in front of the white commissure, 

 it is frequently deflected to one side of the mid-line and presents a slight expansion. 



The separation into halves is completed by the posterior median septum 

 (septum medianum posterius), the so-ca.\\ed pos^en'or median Jissure. With the ex- 

 ception of a shallow groove in the upper cervical cord, the lumbar enlargement and 

 the conus meduUaris, no fissure exists, but in its place a dense partition extends from 

 the posterior surface to the middle of the interior of the cord, ending in close relation 

 to the gray commissure. 



The character of the septum is a subject of dispute, according to some anatomists con- 

 sisting exclusively of condensed neuroglia, while others regard it as composed of pial tissue 

 blended with the neuroglia and, therefore, of both mesoblastic and ectoblastic origin. The 

 latter view is substantiated by the mode of development of the posterior septum, the immature 

 pial covering of the developing blood-vessels being imprisoned within and fused with the neu- 

 rogliar partition derived from the expanding dorsal halves of the developing cord (page 1050). 

 The application of differential stains also demonstrates the composite nature of the septum. 



Fig. 884. 



Each half of the spinal cord is further subdivided by the lines along which the 

 root-fibres of the spinal nerves are attached. The root-line of the dorsal (sensory) 

 fibres is relatively straight and narrow, and marked by a slight furrow, the postero- 

 lateral sulcus (sulcus lateralis posterior) that lies from 2.5-3.5 "^n"^- lateral to the 

 posterior septum and is evident even on the intersegmental intervals where the root- 

 fibres are practically absent. The ventral root-line, marking the emergence of the 

 anterior (motor) fibres, is much less certain, since the bundles of fibres of the indi- 

 vidual nerves do not emerge in the same vertical plane, but overlie one another to , 

 some extent, so that each group occupies a crescentic area, whose greatest width cor- 

 responds in a general way with that of the subjacent ventral horn of gray matter. 

 The anterior root-line, which lies from 2-4 mm. lateral to the median fissure, is 

 neither indicated by a 

 distinct furrow nor con- 

 tinuous. 



In this manner two 

 longitudinal tracts, the 

 posterior columns 

 (funiculi posteriores) are 

 marked off between 

 the posterior median 

 septum and the sulci 

 of the posterior root- 

 lines. These columns 

 include something less 

 than one-third of the 

 semi-circumference of 

 the cord, and are about 

 6 mm. in width in the 

 thoracic cordand 8mm. 

 and 7 mm. in the cervi- 

 cal and lumbar enlarge- 

 ments respectively. 

 The tracts included 



between the dorsal and ventral root-lines constitute the lateral columns (funiculi 

 laterales) and those between the ventral root-lines and anterior median fissure are the 

 anterior columns (funiculi anteriores). Such subdivision into anterior and lateral 



skui: 



Vertebral 

 artery 



1 cerv. nerve 



Atlas 



2 cerv. nerve 



Edge of 

 cut dura 



Spinal acces- 

 sory nerve 



Cut pedicle of 



third cer\'. 



vertebra 



4 cerv. nerve 



Dorsal sur- 

 . face of cord 



Medulla 



Dorsal 

 root-bundles 



Spinal 

 ganglion 



Spinal acces- 

 sorv nerve 



Ganglion 

 on 4 nerve 



Dorsal roots of =; cerv. nerve 



Upper end of spinal cord, viewed from behind after partial removal of dnral 

 sheath ; cord-segments are indicated by groups of converging bundles of posterior 

 root-fibres ; spinal ganglia are seen lying within the intervertebral foramina ; 

 spinal accessory nerve is seen ascending on each side. 



