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NEUROLOGY 



sheath, separated from the pia mater by a comparatively wide interval, the sub- 

 arachnoid cavity, which is filled with cerebrospinal fluid. The pia mater closely 

 invests the medulla spinalis and sends delicate septa into its substance; a narrow 

 band, the ligamentum denticulatum, extends along each of its lateral surfaces 

 and is attached by a series of pointed processes to the inner surface of the dura 

 mater. 



Thirty-one pairs of spinal nerves spring from the medulla spinalis, each nerve 

 having an anterior or ventral, and a posterior or dorsal root, the latter being dis- 

 tinguished by the presence of an oval swelling, 

 the spinal ganglion, w r hich contains numerous 

 nerve cells. Each root consists of several 

 bundles of nerve fibers, and at its attachment 

 extends for some distance along the side of 

 the medulla spinalis. The pairs of spinal 

 nerves are grouped as follows: cervical 8, 

 thoracic 12, lumbar 5, sacral 5, coccygeal 1, 

 and, for convenience of description, the 

 medulla spinalis is divided into cervical, 

 thoracic, lumbar and sacral regions, corre- 

 sponding with the attachments of the different 

 groups of nerves. 



Although no trace of transverse segmen- 

 tation is visible on the surface of the medulla 

 spinalis, it is convenient to regard it as being 

 built up of a series of superimposed spinal 

 segments or neuromeres, each of which has 

 a length equivalent to the extent of attach- 

 ment of a pair of spinal nerves. Since the ex- 

 tent of attachment of the successive pairs of 

 nerves varies in different parts, it follows that 

 the spinal segments are of varying lengths; 

 thus, in the cervical region they average about 

 13 mm., in the mid-thoracic region about 26 

 mm., while in the lumbar and sacral regions 

 they diminish rapidly from about 15 mm. at 

 the level of the first pair of lumbar nerves to 

 about 4 mm. opposite the attachments of the 

 lower sacral nerves. 



As a consequence of the relative inequality 

 in the rates of growth of the medulla spinalis 

 and vertebral column, the nerve roots, which 

 in the early embryo passed transversely out- 

 ward to reach their respective intervertebral foramina, become more and more 

 oblique in direction from above downward, so that the lumbar and sacral nerves 

 descend almost vertically to reach their points of exit. From the appearance these 

 nerves present at their attachment to the medulla spinalis and from their great 

 length they are collectively termed the cauda equina (Fig. 662). 



The filum terminale is a delicate filament, about 20 cm. in length, prolonged 

 downward from the apex of the conus medullaris. It consists of two parts, an upper 

 and a lower. The upper part, or filum terminale internum, measures about 15 cm. 

 in length and reaches as far as the lower border of the second sacral vertebra. It 

 is contained within the tubular sheath of dura mater, and is surrounded by the 

 nerves forming the cauda equina, from which it can be readily recognized by its 

 bluish-white color. The lower part, or filum terminale externum, is closely invested 





FIG. 661. Sagittal section of vertebral canal 

 to show the lower end of the medulla spinalis 

 and the filum terminale. Li, Lv. First and fifth 

 lumbar vertebrae. Sii. Second sacral vertebra. 

 1. Dura mater. 2. Lower part of tube of 

 dura mater. 3. Lower extremity of medulla 

 spinalis. 4. Intradural, _ and 5, Extradural por- 

 tions of filum terminale. 6. Attachment 

 of filum terminale to first segment of coccyx. 

 (Testut.) 



