734 THE NERVOUS SYSTEM 



araclmoid, Init receive funnel-shajx'd sheatlis from it (Key and Retzius). The 

 highest of these processes is at the level of the foramen magnum, and is placed 

 between the hypoglossal nerve and the vertebral artery. In the upper region of 

 the cord one denticulation is regularly placed between the point of exit of each 

 nerve and the next nerve below; lower down the arrangement becomes less regular. 

 It is continuous below with the filum terminale. The linea splendens (Haller) 

 is a conspicuous linear thickening which is placed in the middle line in front of the 

 cord. It })resents a silvery white appearance. It is continuous below with the 

 filum terminale. The filum terminale is the continuation of the pia-matral sheath 

 of the cord, and contains l)ut little nervous matter in its interior. It is easily 

 distinguished by its shining Avhite appearance from the nerve-roots among which it 

 lies. It is a slender strand which extends from the pointed lower extremity of the 

 spinal cord to the lower end of the sacrum, or first piece of the coccyx, opposite 

 which it is attached to the bone. As already mentioned, it receives a sheath from 

 the dura mater in the lower part of its course. 



EXTERNAL CHARACTERS OF THE SPINAL CORD 



The spinal cord is about eighteen inches in length, and forms (l>y weight) about 

 two per cent, of the cerebro-spinal axis. It is cylindrical in form, and is slightly 

 compressed from before backwards, so that the transverse exceeds the antero- 

 posterior diameter. It is continuous with the medulla oblongata above at tlie 

 decussation of the pyramids; below it tapers off into a cone, the conus medul- 

 laris, the apex of which is continued dowuAvards by the filum terminale. It 

 follows the curvatures of the canal in which it is placed, and therefore describes 

 two curves, the upper or cervical convex forwards, and the lower or dorsal concave 

 forwards. 



In two regions of the cord distinct enlargements are visible: these are called 

 the cervical and the luml^ar enlargements. The large size of the nerves Avhich are 

 given off to supply the limbs, forming the brachial and lumbo-sacral plexuses, 

 obviously account for these enlargements of the cord. The increase in size is 

 almost entirely produced by an increase in the transverse diameter. The cervical 

 enlargement is more pronounced than the lumbar swelling. It commences at 

 the level of the third cervical vertebra, and ends at the second dorsal. It is at its 

 maximum at the level of the sixth cervical vertebra. The lumbar enlargement 

 commences at the level of the ninth dorsal vertebra, and reaches its maximum 

 at the twelfth dorsal; below this point it rajiidly diminishes and passes into the 

 conus medullaris. 



The spinal cord is bilaterally symmetrical, the two halves being defined by an 

 anterior fissure and a posterior septum. From the antero-lateral and postero- 

 lateral aspects of the cord the anterior and posterior nerve-roots emerge, forming 

 two parallel series of fasciculi which extend throughout the entire length of the 

 cord. In the upper cervical region the intrathecal course of the nerve-roots is very 

 short, and the direction taken by the roots is nearly transverse. As the roots are 

 traced downwards in the dorsal and lumbar regions, they become progressively 

 longer, and descend with an increasing degree of obliquity. Hence the lumbar and 

 sacral nerve-roots have a very long intrathecal course. They extend beyond the 

 tern)ination of the cord (which, it will be remembered, ends at the body of the 

 second lumbar verteln-a), and lie within the theea in a sheaf somewhat resembling 

 a horse's tail, and therefore called the cauda equina. 



The cord is divided into definite macroscopical areas l:>y one fissure, two sulci, 

 three septa, and the exits of the anterior nerve-roots. 



The fissure. — The anterior fissure separates the cord into two lateral areas 

 anteriorly (fig. 439). It is relatively wide and extends from the surface to the 

 anterior white commissure, attaining a depth equal to about one-third the antero- 

 posterior diameter of the cord. It contains a fold of pia mater and l)ranehes of the 

 anterior spinal vessels. It is continued above along the anterior surface of the 

 medulla, being partially interrupted by the decussation of the pyramids, and it 

 terminates at the foramen caecum just below the pons (fig. 441). 



