SPINAL CORD. 433 



fissura longitudinalis posterior. It extends more deeply into the cord 

 than the anterior fissure, and terminates in the grey substance of the 

 interior. These two fissures divide the medulla spinalis into two 

 lateral cords, which are connected to each other by the white com- 

 missure which forms the bottom of the anterior longitudinal fissure, 

 and by a commissure of grey matter situated behind the former. On 

 either side of the fissura longitudinalis posterior is a slight line, which 

 bounds on each side the posterior median columns. These columns 

 are most apparent at the upper part of the cord, near the fourth 

 ventricle, where they are separated by the point of the calamus scrip- 

 torius, and where they form a bulbous enlargement at each side, 

 called the processus clavatus. 



Two other lines are observed on the medulla, the anterior and pos- 

 terior lateral sulci, corresponding with the attachment of the anterior 

 and posterior roots of the spinal nerves. The anterior lateral sulcus 

 is a mere trace, marked only by the attachment of the filaments of the 

 anterior roots. The posterior lateral sulcus is more evident, and is 

 a narrow greyish line derived from the grey substance of the interior. 



Although these fissures and sulci indicate a division of the spinal 

 cord into three pairs of columns, namely, anterior, lateral, and poste- 

 rior, the posterior median columns being regarded as a part of the 

 posterior columns, it is customary to consider each half of the spinal 

 cord as consisting of two columns only, the antero-lateral and the 

 posterior. The antero-lateral columns are the columns of motion, and 

 comprehend all that part of the cord situated between the fissura 

 longitudinalis anterior and the posterior lateral sulcus, the grey line of 

 origin of the posterior roots of the spinal nerves. The posterior 

 columns are the columns of sensation. 



If a transverse section of the spinal cord be made, its internal struc- 

 ture may be seen and examined. It would then appear to be com- 

 posed of two hollow cylinders of white matter, placed side by side, 

 and connected by a narrow white commissure. Each cylinder is filled 

 with grey substance, which is connected by a commissure of the same 

 matter. The form of the grey substance, as observed in the section, is 

 that of two half-moons placed back to back, and joined by a transverse 

 band. The horns of the moons correspond to the sulci of origin of the 

 anterior and posterior roots of the nerves. The anterior horns, larger 

 than the posterior, do not quite reach this surface ; but the posterior 

 appear upon the surface, and form a narrow grey line, the sulcus 

 lateralis posterior. 



The white substance of the spinal cord is composed of parallel fibres 

 which are collected into longitudinal laminae and extend throughout 

 the entire length of the cord. These laminae are various in breadth, 

 and are arranged in a radiated manner ; one border being thick arid 

 corresponding with the surface of the cord, while the other is thin and 

 lies in contact with the grey substance of the interior. According to 

 Rolando the white substance constitutes a simple nervous membrane, 

 which is folded into longitudinal plaits, having the radiated disposition 



2 F 



