) 



776 THE NERVOUS SYSTEM 



canal extending a short distance into the filum terminale, and there ends blindly. 

 The grey commissure always lies somewhat nearer the ventral than the dorsal 

 surface of the cord, and itself contains a few meduUated axones which vary in 

 amount in the different regions of the cord. The meduUated axones crossing the 

 mid-line on the ventral side of the central canal form the ventral or anterior white 

 commissure ; those, usually much fewer in number, crossing on the dorsal side of 

 the central canal, form the dorsal or posterior white commissure. These two 

 commissures comprise fibres crossing in the grey substance as distinguished from 

 others which cross in the white substance dorsal and ventral to them. The axones 

 of these commissures serve in functionally associating the two lateral halves of the 

 grey column. 



Each lateral half of the gre}^ column presents a somewhat crescentic or comma- 

 shaped appearance in transverse section, which also varies at the different levels 

 of the cord. At all levels each half presents two vertical, well-defined horns, 

 themselves spoken of as columns of grey substance. The dorsal horn [columna 

 posterior] extends posteriorly and somewhat laterally toward the surface of the 

 cord along the line of the postero-lateral sulcus. It is composed of an apex and a 

 neck [cervix columnse posterioris]. 



In structure the apex is peculiar. The greater portion of it consists of a mass of small 

 nerve-cells and neuroglia tissue, among which a gelatinous substance of questionable origin 

 predominates, giving the horn a semi-translucent appearance. This is termed the gelatinous 

 substance of Rolando, to distinguish it from a similar appearance immediately about the central 

 canal, the central gelatinous substance. The apex of the dorsal horn is widest in the regions 

 of the enlargements, especially the lumbar, and the gelatinous substance of Rolando is most 

 marked in the cervical region. In these regions the cervix consists of a slight constriction of 

 the dorsal horn between the apex and the line of the grey commissure. In the thoracic region, 

 however, the base of the cervix is the thickest part of the dorsal horn. This thickness is due to 

 the presence there of the nucleus dorsalis, or Clarke's column — a column of grey substance 

 containing numerous nerve-cells of larger size than elsewhere in the dorsal horn, and extending 

 between the seventh cervical and third lumbar segments of the cord. Tapering finely at its 

 ends, this nucleus attains its height in the lower thoracic or first lumbar segment. About the 

 ventro-lateral periphery of the nucleus dorsalis are scattered nerve-cells of the same type as 

 contained in it. These cells are sometimes distinguished as Stilling's nucleus, though Clarke's 

 column was also described by Stilling. They are more numerous about the lower extremity 

 of the nucleus dorsalis, and they continue to appear below its termination in the lumbar region. 



The ventral horn [columna anterior] of each lateral half of the grey figure is 

 directed ventrally toward the surface of the spinal cord, pointing toward the 

 antero-lateral sulcus. It contains the cell-bodies which give origin to the efferent 

 or ventral root axones, and these axones make their emergence from the spinal 

 cord along the antero-lateral sulcus. The ventral horns vary markedly in shape 

 in the different regions. In certain segments each ventral horn is thickened later- 

 ally and thus presents its two component columns of grey substance: the lateral 

 horn [columna lateralis], a triangular projection of grey substance into the 

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

 commissure; and the ventral horn proper [columna anterior], projecting ventrallj'-. 

 In the mid-thoracic region the lateral horn is relatively insignificant, and the 

 anterior horn is quite slender; in the cervical and lumbar enlargements both horns 

 are considerably enlarged. 



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

 blending of the two there are often 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 trabeculss are most 

 marked along the lateral aspects of the grey figure and present there the appear- 

 ance known as the reticular formation. The reticular formation of the spinal 

 cord is most evident in the cervical region (fig. GIG). 



Minute structure. — The large cell-bodies of the Ventral horn as a whole are divisible into 

 four groups, only three of which are to be distinguished in the mid-thoracic region of the spinal 

 cord: — (1) A ventral group of cells, sometimes separated into a ventro-lateral and a ventro- 

 medial portion (see figs. (JIG, 619), occupies the ventral horn proper, is constant throughout 

 the entire length of the cord, and contributes axones to the ventral root, most of which probably 

 supply the muscles adjacent to the vertebral column; (2) a dorso-medial group of cells, situated 

 in the medial part of the vtuitral horn, just below the level of the central canal, gives origin to 

 axones some of which go to tlu; ventral root of the same side, but most of which cross the mid- 

 line viA, the anterior wliitc commissure, either to pass out in the ventral root of the opposite 

 side or to enter the white substance of that side and course upward or downward, associating 

 with other levels of the cord. Some of its axones terminate among the cells of tiie ventral horn 



