THE NERVOUS SYSTEM 1315 



fissure and the most external fasciculi of the anterior nerve-roots ; 

 the lateral column is the area between the most external fasciculi of 

 the anterior nerve-roots and the postero -lateral sulcus ; and the 

 posterior column lies between the postero-lateral sulcus and the 

 posterior median fissure. Practically the anterior colrmin repre- 

 sents the region in front of the anterior nerve-roots, the lateral 

 column the region bet^^'een the anterior and posterior nerve-roots, 

 and the posterior colmnn the region behind the posterior nerve-roots. 

 According to some authorities there are only two columns, namely, 

 antero-lateral and posterior, the former extending from the anterior 

 median fissure to the postero-lateral sulcus, and representing the 

 combined anterior and lateral colimms. 



In the cervical region the siurface of each posterior colmim 

 presents a shght groove which is situated nearer the posterior 

 median fissure than the postero-lateral sulcus. This groove is called 

 the posterior intermedial or paramedian furrow. It contains a 

 septimi of pia mater, and in this manner the posterior column of the 

 cord is marked off into two tracts. The inner and smaller tract is 

 called the postero-median column, or column of GoU, and the outer 

 and larger is called the postero-extemal column, or column ol 

 Burdach. These two columns extend throughout the cord, but it 

 is only above the level of the mid-thoracic region that they are 

 separated from each other by a septum of pia mater, known as the 

 posterior ititermediate septum. 



Origin of the Spinal Nerves. — • There are thirty-one pairs of 

 spinal nerves, which arise from the sides of the spinal cord. They 

 are arranged in five groups, on either side, as follows: cervical, 

 eight in number ; thoracic, twelve ; lumbar, five ; sacral, five ; and 

 coccygeal, one. Each spinal nerve is attached superficially to the 

 cord by two roots, anterior and posterior, the posterior root being 

 the larger of the tw-o. The portion of the cord from which each 

 pair of spinal nerves arise is spoken of as a segment of the cord. 

 Each root is ensheathed by tubular prolongations of the coverings 

 of the cord, namely, the pia mater, arachnoid, and dura mater, in 

 this order from within outwards, and these sheaths ultimately 

 blend with the perineurium. The roots are separated from each 

 other by the lateral colimm of the cord and the ligamentum 

 denticulatimi, and l^ey pass through separate openings in the theca 

 of dura mater. 



The anterior roots are composed of efferent or motor fibres, and 

 their fasciculi emerge from the cord in an irregular manner, being 

 spread over an area corresponding in breadth to the caput of 

 the anterior comu of the grey matter in the interior. The 

 posterior roots are composed of afferent or sensory fibres, and their 

 fasciculi enter the cord in a straight line along the course of 

 the postero-lateral sulcus. Each posterior root presents an oval 

 swelling, called the spinal ganglion. These gangUa are, for the 

 most part, situated in tiie intervertebral foramina, and immediately 

 beyond each ganglion the anterior and posterior roots unite to form 



