THE SPINAL CORD. 



267 



moreover, is not uniform on account of two fusiform swellings, the cervical 

 and lumbar enlargements, associated with the origin and reception of the 

 large nerves supplying the limbs. Where least expanded, opposite the 

 middle of the thoracic spine, the cord measures 8 mm. in its sagittal and 

 10 mm. in its transverse diameter. Through the cervical enlargement these 

 respective dimensions are 9 mm. and 14 mm., and through the lumbar 

 swelling they are 8.5 mm. and 12 mm. 



Cross-sections of the spinal cord (Fig. 313) show it to be imperfectly 

 divided into symmetrical halves by a narrow cleft, the anterior median fissure, 

 in front and a partition, the posterior -median septum, behind. Further, the 

 cord is seen, even with the unaided eye, to be composed of an irregular 

 H -shaped core of gray substance enclosed by a mantle of white matter. 



Nerve-fibres of white matter Anterior root-fibres 



FIG. 314. Portion of anterior horn of gray matter, showing multipolar nerve-cells and root-fibres. X "Q 



The latter, in each half of the cord, is partially subdivided into three general 

 tracts by the lines along which the root-fibres of the spinal nerves are attached. 

 The dorsal root-line of the sensory fibres is marked by a slight furrow, the 

 postero-lateral sulcus, that lies from 2.5-3.5 mrn - lateral to the posterior 

 median septum. The ventral root-line, marking the emergence of the 

 anterior (motor) fibres, is much less evident on account of the scattered 

 manner in which these root-fibres make their exit. / In this manner three 

 longitudinal tracts, the columns of the cord, ace marked off on each 

 side the posterior between the median septum and tile postero-lateral sulcus, 



