TllK SI'I.\.\L CORD 7(i:; 



the sulcus. The ventral horns vary markedly In shape in the different regions. In 

 certain segments each ventral horn clearly presents its two component columns of 

 grey substance: the lateral horn (colnnum liitirul/x). a triangular projection of 

 grey substance into the surrounding white substance, in line with or a little ventral 

 to t 'lie line of t he grey commissure : and t he ventral horn proper (cohintini u/ili rior), 

 projecting ventrally. In the mid-thoracic region the lateral horn is relatively insig- 

 nificant, and the anterior horn is quite slender: in the cervical and lumbar enlarge- 

 ments both horns are considerably enlarged and fuse into one large column. 



The grey substance is not sharply demarcated from the white. In the blending 

 of the two there often are small fasciculi of white substance embedded in the grey, and 

 likewise the grey substance sends fine processes among the axones composing the 

 white substance. Such processes or grey trabeciihe are most marked along the 

 lateral aspects of the grey figure and present there the appearance known as the 

 reticular formation. The reticular formation of the spinal cord is most evident 

 in the cervical region (fig. 569). 



The large cell-bodies of the ventral horn as a whole are divisible into four groups, 

 only two of which are to be distinguished in the mid-thoracic region of the spinal 

 (., ,n| : (i) A fi'iitnil i/roup of cells, sometimes separated into a ventro-lateral and a 

 ventro-medial portion (see figs. 569 and 571), occupies the anterior column, 

 is constant throughout the entire length of the cord, and contributes axones 

 to the ventral root, most of which probably supply the muscles proximal to the spinal 

 column; (2) a ilorxo-niulinl group of cells, situated in the medial part of the ventral 

 horn, just below the level of the central canal, gives origin to axones some of which 

 go to the ventral root of trfe same side, but most of which cross the mid-line via the 

 anterior white commissure, either to pass out in the ventral root of the opposite side 

 or to enter the white substance of that side and course upwards or downwards, asso- 

 ciat ing with other levels of the cord. Some of its axones terminate among the cells 

 of the ventral horn in the same level of the opposite side; (3) a lateral group of cells, 

 sometimes separated into a dorso-lateral and a ventro-lateral portion, occupies the 

 Literal column of the horn, and is best differentiated in the cervical and lumbar 

 enlargements. Most of the axones arising from its larger cells are contributed to 

 t he vent ral root of the same side, and such axones probably supply the muscles of the 

 extremities. Some of those from its ventral portion are distributed to the muscles 

 of the body-wall ; (4) an intermediate group, occupying the mid-dorsal portion of the 

 ventral horn. Axones arising from its cells are probably seldom contributed to the 

 ventral root, but instead course wholly within the central nervous system. Some 

 pass to the opposite side of the cord, chiefly via the anterior and possibly the posterior 

 white commissure, to terminate either in the same or different levels of the grey col- 

 umn. ( Hhers of longer course pass to the periphery of the cord, join one of the cere- 

 bello-spinal fasciculi, and pass upwards to the cerebellum. 



Furthermore, there are scattered throughout the grey substance many smaller 

 nerve-cells. These give rise to axones of shorter course, either commissural or 

 associational proper. Of such axones many are quite short, coursing practically 

 in the same level as that in which their cells of origin are located, and serve to con- 

 nect the different parts of the grey substance of that level. Others course varying 

 distances upwards and downwards for the association of different levels of the grey 

 column. 



It is evident from the above that in addition to the various nerve-cells it contains, 

 there is also to be found a felt-work of axones in the grey substance. Many of these 

 axones are medullated, though not in sufficient abundance to destroy the grey char- 

 acter of the substance. The felt-work is composed of three general varieties of fibres: 

 (1) The terminal twigs of axones and their collaterals entering from the fasciculi 

 of the white substance and forming end-brushes about the various cell-bodies in the 

 grey substance (partly medullated); (2) axones given off from the cells of the grey 

 substance and which pass into the surrounding white substance either to enter the 

 ventral roots or to join the ascending and descending fasciculi within the spinal cord 

 (partly medullated) ; (3) axones of C.olgi cells of typo 1 1. which do not pass outside 

 the confines of the grey substance (non-medulla ted). Some axones of any of these 

 varieties may cross the mid-line and thus become commissural. In general all fibres 

 of long course acquire medullary sheaths a short distance from their cells of origin, 

 and lose them again just before termination. 



The white substance of the spinal cord. The great mass of the medullated 



