74 



THE NERVOUS SYSTEM 



from which the nerves of the brachial plexus arise, that is, the fourth cervical 

 to the second thoracic segments inclusive. The lumbar enlargement (intumes- 

 centia lumbalis) is not quite so extensive and corresponds less accurately to the 

 origin of the nerves innervating the lower extremity. At an early stage in the 

 embryonic development of the spinal cord these enlargements are not present. 

 In the time of their first appearance and in their subsequent growth they are 

 directly related to the development of the limbs. 



Below the lumbar enlargement the spinal cord rapidly decreases in size 

 and has a cone-shaped termination, the conus medullaris, from the end of which 

 a slender filament, the filum terminale, is prolonged to the posterior surface of 

 the coccyx (Figs. 50, 51). This terminal filament descends in the middle line, 

 surrounded by the roots of the lumbar and sacral nerves, to the caudal end of 



Septum poslicum 



/Posterior spinal artery 



_,,,... , , ^^>_- Ligamentum denticulatum 



Subarachnoid trabecuke ---.-. 



Pia mater - 



Epidural trabeculce *=;2?5 

 Anterior spinal artery' 



--Dura mater 

 *- Subdural space 

 Arachnoid 

 '-Nerve root 



Subarachnoid cavity 

 Linca splendens 

 Fig. 49. Diagram of the spinal cord and meninges. 



the dural sac at the level of the second sacral vertebra. Here it perforates the 

 dura mater, from which it receives an investment, and then continues to the 

 posterior surface of the coccyx. The last portion of the filament with its dural 

 investment is often called the filum of the spinal dura mater (filum durae matris 

 spinalis). The filum terminale is composed chiefly of pia mater; but in its 

 rostral part it contains a prolongation of the central canal of the cord. 



The spinal cord shows an obscure segmentation, in that It gives origin to 

 thirty-one pairs of metameric nerves. These segments may be somewhat 

 arbitrarily marked off from each other by passing imaginary planes through the 

 highest root filaments of each successive spinal nerve (Donaldson and Davis, 

 1903). The highest of these planes, being just above the origin of the first cer- 

 vical nerve, marks the separation of the spinal cord from the medulla oblongata. 



