CHAP, i.] THE SPINAL CORD. 689 



appears, the fibres forming it, crossing over in the anterior com- 

 missure, pass to the nerve-cells of the anterior horn of the 

 opposite side with which, like the brother-fibres of the crossed 

 pyramidal tract, they make connections by contact. 



A conspicuous tract in the white matter of the lateral column 

 is the cerebellar tract (Figs. 113, 114) marked out by a descend- 

 ing degeneration. This, starting in the lower parts of the cord, 

 and as a whole increasing as it goes, may be traced through the 

 restiform body of the same side to the cerebellum. We have 

 reason to believe that the cells, the axis-cylinder processes of 

 cells which supply the fibres composing the tract, are those form- 

 ing the posterior vesicular cylinder. The tract, we may con- 

 clude, carries to the cerebellum afferent impulses furnished by 

 the posterior roots, but modified, we may presume, at the relay in 

 the posterior vesicular cylinder. 



Other tracts have also been made out in the white matter of 

 the cord, but these are not so conspicuous as the above. It is 

 more important to remember that a large number of the fibres, 

 both of the lateral and anterior (ventral) columns are axis- 

 cylinder processes of cells of one part of the cord, and end by 

 arborescences making contact with cells in another part of the 

 cord. Some of these fibres run their whole course on the same 

 side of the cord ; others cross over to the opposite side ; these 

 may be regarded as commissural fibres, longitudinal and trans- 

 verse. Lastly, some of the cells in the grey matter are such 

 that all their processes end as they begin, within the grey 

 matter, and do not contribute at all to the white matter. 



453. The Special Features of the several regions of the 

 Spinal Cord. The cord begins below in the slender filament 

 called the filum terminable, 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 begin- 

 ning 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 enlarge- 

 ment, 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 



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