MICROSCOPICAL STRUCTURE OF SPINAL CORD. 1033 



The ventro-median cell-column is the most constant, since, as emphasized by the pains- 

 taking studies of Bruce,' it is interrupted only between the levels of the fifth lumbar and first 

 sacral nerve in its otherwise unbroken course through the length of the cord, as far as the level 

 of the fifth sacral nerve. An augmentation of this tract in the fourth and fifth cervical segments 

 is probably associated with the spinal origin of the phrenic nerve (Bruce). 



The dorso-mesial cell-column is much less constant, being represented only in the thoracic 

 region, in a few cervical segments and at the level of the first lumbar nerve. In agreement 

 with van Gehuchten and others, Bruce regards the continuity of the mesial group as presump- 

 tive evidence of its close relation to the dorsal extensor muscles of the trunk. 



The ventro-lateral cell-column appears first at the level of the fourth cervical nerve, 

 increases rapidly in the succeeding segments and fades away at the lower part of the eighth 

 cervical segment. It reappears in the lumbar enlargement, reaching its maximum at the level 

 of the first sacral nerve and, diminishing rapidly through the upper part of the second, 

 disappears before the third sacral segment is reached. 



The dorso-lateral cell-column, in places the most conspicuous collection of the anterior 

 horn, begins above at the lower part of the fourth cervical segment and, increasing rapidly, 

 attains its greatest development in the neck in the fifth and sixth segments. It suffers a marked 

 reduction at the level of the seventh cervical nerve, which is followed by a sudden increase in 

 the next segment in which the column presents an additional collection of nerve-cells known as 

 the accessory dorso-lateral or post-postero- lateral group. Below the level of the second thoracic 

 nerve the dorso-lateral cell-column is unrepresented as far as the second sacral segment where 

 it reappears, somewhat abruptly, and attains its maximum size in the fourth and fifth lumbar 

 segments. The column then diminishes and ceases at the lower part of the third sacral seg- 

 ment. Within the sacral cord, between the levels of the first and third nerve inclusive, the 

 dorso-lateral cell-group is augmented by an accessory group. From the third lumbar to the 

 sacral nerve-levels, an additional compact collection of nerve-cells occupies a more median 

 position in the anterior horn and constitutes the central group. 



From the position of the greatest expansions of the lateral cell-columns — within the cervical 

 and lumbo-sacral enlargements — it is evident that they are associated with the large nerves sup- 

 plying the muscles of the limbs. Further, according to Bruce, in a general way the size of the 

 radicular cells bears a relation to that of the muscles supplied, the smaller dimensions of the 

 cervical cells, as compared with those of the lumbo-sacral region, corresponding with the smaller 

 size of the upper limb in comparison with that of the lower one. 



In addition to the nerve-cells assembled within the foregoing more or less well defined 

 groups, some scattered cells are irregularly distributed through the anterior horn and do not 

 strictly belong to any of the groups. 



Below the level of the first coccygeal nerve, the cells of the anterior horn become so 

 diminished in number, that they are no longer grouped with regularity, but, reduced in size, lie 

 uncertainly distributed within the gray matter as far as the lower limits of the conus medullaris. 



The nerve-cells of the posterior horn are neither as large nor as regularly 

 disposed as the anterior horn cells. Only in one locality, along the median border 

 of the base of the posterior horn, are they collected into a distinct tract, the column of 

 Clarke ; otherwise they- are scattered without order throughout the gray matter of the 

 posterior cornu. Since, however, the latter comprises certain areas, the cells of 

 the posterior horn may be di\-ided into (i) the cells of Clarke' s column, (2) the 

 cells of the substantia gelatinosa Rolandi, and (3) the i?iner cells of the caput cornu. 



The cells of Clarke's column form a very conspicuous collection which extends from the level 

 of the seventh cervical nerve to that of the second lumbar nerve and is best developed in the 

 lower thoracic region of the cord. Although confined chiefly to the dorsal portion of the cord, 

 and hence sometimes designated as the "dorsal nucleus," Clarke's column is represented to a 

 slight degree in the sacral and upper cervical regions {sacral and cervical nuclei of Stilling) . In 

 cross-sections the cell-column appears as a group of multipolar cells that occupy the mesial 

 border of tlie base of the posterior horn and, where the column is best developed (opposite the 

 origin of the twelfth thoracic nerve), correspond to an elevation on the surface of the gray 

 matter. The cells usually are about .050 mm. in diameter, polygonal in outline and possess a 

 relatively large number of richly branched dendrites that radiate chiefly within the limits of the 

 group (Cajal). The axones commonly spring from the anterior or lateral margin of the cells 

 and course ventrally for a considerable distance before bending outward toward the lateral 

 column of white matter within which, as constituent fibres of the direct cerebellar tract 

 (page 1044) , they turn brainward. 



'Topographical Atlas of the Spinal Cord, 1901. 



