518 



THE NEKVOUS SYSTEM. 



At the margin of the foramen magnum the spinal medulla becomes continuous 

 with the medulla oblongata of the brain, whilst below, it tapers rapidly to a 

 point and forms a conical extremity termed the conus medullaris. From the end 

 of the conus medullaris a slender glistening thread is prolonged downwards within 

 the vertebral canal, and finally anchors the spinal medulla to the back of the coccyx. 

 This prolongation receives the name of the filum terminale. 



The diameter of the spinal medulla is very much shorter than that of the 

 vertebral canal within which it lies. A wide interval is left between its surface 

 and the walls of its canal, and this excess of space is clearly a provision for 

 allowing free movement of the vertebral column without producing any jarring 

 contact between the delicate spinal medulla and the surrounding bones. 



Three protective membranes are wrapped around the spinal medulla. From 

 within outwards these are termed (1) the pia mater, (2) the arachnoid, and (3) the 



dura mater. The pia mater is a fibrous 

 membrane which forms the immediate 

 investment. It is closely applied to 

 the spinal medulla, and from its deep 



Conus medullaris 



Posterior lateral 

 groove 



Anterior nerve-root 

 Posterior nerve -root 



FIG. 460. THE CONUS MEDULLARIS AND THE 

 FILUM TERMINALE EXPOSED WITHIN THE 

 VERTEBRAL CANAL. 



Spinal ganglion 



Anterior ramus 

 of spinal nerve 

 Posterior raimis 

 of spinal nerve 



FIG. 461. THE ROOTS OF ORIGIN OF THE 

 SEVENTH THORACIC NERVE (semi- diagram- 

 matic). 



surface numerous fine septa penetrate into the substance of the spinal medulla 

 The arachnoid is an exceedingly delicate transparent membrane which is loosel) 

 wrapped around the spinal medulla so as to leave a considerable interval, betweer 

 itself and the pia mater, termed the subarachnoid space, in which there is always * 

 varying amount of cerebro-spinal fluid. Outside the arachnoid, the dura mater form 

 a wide, dense, fibrous, tubular sheath, which extends downwards within the vertebra 

 canal for a considerable distance beyond the conical extremity of the spinal medulk 

 The spinal medulla is suspended within its sheath or theca of dura mater by tw 

 lateral wing- like ligaments, termed the ligamenta denticulata. These extend lateral! 

 from the sides of the spinal medulla and are attached by a series of pointed c 

 tooth-like processes to the inner surface of the theca of dura mater. Betwee 

 the wall of the vertebral canal and the dura mater there is a narrow interval, whic 

 is filled up by soft areolo- fatty tissue and numerous thin-walled veins arrange 

 in a plexiform manner. 



Thirty-one pairs of spinal nerves arise from the sides of the spinal medull 



