878 THE NERVE ROOTS. [BOOK in. 



and into a still more lateral bundle (Fig. 99, Pr"). The former, 

 consisting also of coarse fibres, plunges directly through the sub- 

 stance of Rolando at the extremity of, and so into the grey matter 

 of the horn, where the fibres changing their direction run in part 

 at least longitudinally in the grey matter in bundles known as 

 "the longitudinal bundles of the posterior horn" Figs. 98, 99 

 r. f. p. some of which appear to pass on to the anterior horn. 

 The small most external or lateral portion of the lateral bundle, 

 consisting of fine fibres and sometimes spoken of as the lateral 

 bundle, on entering the cord at once ascends for some distance, 

 and thus forms the thin layer of fine fibres, the posterior marginal 

 zone or Lissauer's zone, indicated in Fig. 99 by ra. t. t which lies 

 between the actual extremity of the horn and the surface of the 

 cord, and in the upper regions of the cord (cf. Fig. 98, p') runs 

 some way upward on the lateral margin of the horn between the 

 grey matter and the crossed pyramidal tract. As it ascends this 

 layer continually gives off fibres to the grey matter of the 

 posterior horn in the cells of which they appear to end. 



Thus, while part of the median bundle does not join the grey 

 matter at all but goes to form the median posterior tract, the rest 

 of that bundle and all the other fibres of the root, sooner or later, 

 join the grey matter either of the posterior horn or of some other 

 part. 



570. The Special Features of the several regions of the Spinal 

 Cord. The cord begins below in the slender filament called the 

 filum terminale, which lying in the vertebral canal, in the midst of 

 the mass of nerve roots called the cauda equina, rapidly enlarges 

 at about the level of the first lumbar vertebra into the conus 

 medullaris. This may be regarded as the beginning of the lower 

 portion of a fusiform enlargement of the cord known as the lumbar 

 swelling, which reaches as high as about the attachment of the 

 roots of the twelfth or eleventh thoracic nerve at the level of the 

 eighth thoracic vertebra, the broadest part of the swelling being 

 about opposite the third lumbar nerve. Above the lumbar 

 swelling, through the thoracic region the somewhat narrowed cord 

 retains about the same diameter until it reaches the level of 

 the first or second thoracic nerve opposite the seventh cervical 

 vertebra where a second fusiform enlargement, the cervical swelling, 

 broader and longer than the lumbar swelling, begins. The broadest 

 part of the cervical swelling is about opposite to the fifth or sixth 

 cervical nerve ; from thence the diameter of the cord becomes 

 gradually somewhat less until it begins to expand into the bulb, 

 but even in the highest part is greater than in the thoracic region. 

 The sectional area of the cord increases therefore from below 

 upwards, but not regularly, the irregularity being due to the 

 lumbar and cervical swellings. 



The extremity of the filum terminale is said to consist entirely 

 of neuroglia closely invested by the membranes, even the central 



