526 THE THORAX AND VERTEBRAL COLUMN 



On transverse section (Fig. 161) it is seen that the grey 

 matter, on each side, is arranged in two columns. The anterior 

 column is broad and blunted and contains the motor cells which 

 give origin to the fibres of the anterior nerve-roots. The posterior 

 column, which is continuous in front with the anterior column, 

 is somewhat pointed, and near its tip the posterior nerve-roots 

 enter the spinal medulla. 



The anterior nerve-roots, which are purely motor, emerge 

 from the anterior column of grey matter in series. The posterior 

 nerve-roots, which are entirely sensory, enter the spinal medulla 



FIG. 161. Diagram of a Transverse Section through the Spinal Medulla. 

 The anterior (motor) and the posterior (sensory) nerve-roots ; their union 

 to form a typical spinal nerve and their subsequent division into anterior 

 and posterior rami are also shown. 



in series on its postero-lateral aspect. Each posterior nerve-root 

 possesses a ganglion, the cells of which give origin to peripheral 

 and central fibres. In or near the inter vertebral foramina the 

 anterior and posterior nerve-roots unite to form a spinal nerve 

 (Fig. 161), and, almost as soon as it is formed, each spinal nerve 

 breaks up into an anterior and & posterior ramus (primary division). 

 The rami, both anterior and posterior, are mixed nerves ; their 

 distribution is dealt with in the various regions of the body. 



Membranes. The spinal medulla is surrounded by three 

 membranous sheaths, which are continuous with the membranes 

 of the brain (p. 223). The pia mater is closely applied to the 

 spinal medulla and, at its lower end, is continued down as a fine 



