858 STRUCTURE OF SPINAL CORD. [BOOK in. 



anterior fissure, and ought to be called a septum rather than a 

 fissure. Between the two fissures the substance of the cord is 

 reduced to a narrow isthmus uniting the two lateral halves, which 

 in a normal cord are like each other in every respect. In the 

 middle of the isthmus lies the section of a small canal, the central 

 canal (Fig. 96, c. c.), which is all that remains of the relatively 

 wide neural canal of the embryo. 



Each lateral half consists of an outer zone of white matter 

 surrounding, except at the isthmus, an inner more or less 

 crescentic, or comma shaped mass of grey matter. The convexity 

 of each crescent is turned towards the median line of the cord, the 

 two crescents being placed back and back and joined together 

 by the isthmus just spoken of. The somewhat broader anterior 

 extremity of the crescent, or head of the comma, is called the 

 anterior cornu or horn ; and the narrower posterior extremity of 

 the crescent, or tail of the comma, is called the posterior cornu or 

 horn. The part by which each horn is joined on to the middle 

 part of the crescent is called the cervix, anterior and posterior 

 respectively. The isthmus joining the backs of the two crescents, 

 like the crescents themselves, consists, for the most part, of grey 

 matter, the band running posterior or dorsal to the central canal 

 being called the posterior grey commissure (Fig. 96, p. g. c), and 

 the band running anterior or ventral to the canal being called the 

 anterior grey commissure (Fig. 96, a. g. c). The posterior fissure 

 touches the posterior grey commissure, but the anterior grey 

 commissure is separated from the bottom of the anterior fissure 

 by a band of white matter, called the anterior white commissure 

 or, more simply, the white commissure or sometimes the anterior 

 commissure (Fig. 96, a. c). 



If the section be taken at the level of the origin of a pair of 

 spinal nerves, it will be seen that the anterior or ventral root, 

 piercing the white matter opposite the head of the comma in 

 several distinct bundles (Fig. 96, A.r), plunges into the anterior 

 cornu, while the posterior or dorsal root (Fig. 96, P.r, P.r'}, having 

 the appearance of a single undivided bundle, passes, in part at 

 least, into the posterior horn. Both roots are dispersed length- 

 ways along the cord, the hinder roots of one nerve being close to 

 the foremost roots of the nerve below, but it is only the anterior 

 roots which are dispersed sideways. The compact bundle of the 

 posterior root divides, with tolerable sharpness, the white matter 

 in each lateral half of the cord into a posterior portion lying 

 between the posterior fissure and the posterior root (Fig. 96 post, 

 col.}, which portion since, as we shall see, it runs, in the form 

 of a column along the length of the cord, is called the posterior 

 column, and into a portion lying to the outside of the posterior 

 root between it and the anterior fissure, called the antero- 

 latdral column. This latter may be considered as further divided, 

 by the entrance of the anterior roots into a lateral column (Fig. 96, 



