762 THE NERVOUS SYSTEM 



in which cell-bodies of neurones, non-medullated axones, and supporting tissue pre- 

 dominate have a darker or greyish appearance, and are known as grey substance. 

 The spinal cord consists of a continuous, centrally placed column of grey substance 

 surrounded by a variously thickened tunic of white substance. The closely invest- 

 ing pia mater sends numerous ingrowths into the cord, bearing blood-vessels and 

 contributing to its internal supporting tissue. The volume of white and of grey 

 substance varies both absolutely and relatively at different levels of the cord. The 

 absolute amount of grey substance increases with the enlargements. The absolute 

 amount of white substance also increases with the enlargements coincident with the 

 greater amount of grey substance in those regions. The relative amount of white 

 substance increases in passing from the conus medullaris to the medulla oblongata, 

 due to the fact that the ascending and descending axones connecting the cord with 

 the encephalon are contributed at different levels of the cord along its entire course. 



The grey substance. In the embryo all the nerve-cells of the grey substance 

 are derived from the cells lining the neural tube, and in the adult the column of grey 

 substance, though greatly modified in shape, still retains its position about the cen- 

 tral canal. In transverse section the column appears as a grey figure of two laterally 

 developed halves, connected across the mid-line by a more attenuated portion, the 

 whole roughly resembling the letter H. The cross-bar of the H is known as the grey 

 commissure. Naturally, it contains the central canal, which is quite small and 

 is either rounded or laterally or ventrally oval in section, according to the level of 

 the cord in which it is examined. The canal continues upwards, and in the medulla 

 oblongata opens out into the fourth ventricle. Downwards, in the extremity of 

 the conus medullaris, it widens slightly and forms the rhomboidal sinus or termi- 

 nal ventricle, then is suddenly constricted into an extremely small canal extending 

 a short distance into the filum terminale, and there ends blindly. The grey com- 

 missure always lies somewhat nearer the ventral than the dorsal surface of the cord, 

 and itself contains a few medullated axones which vary in amount in the different 

 regions of the cord. Of these, the 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. The axones of these commissures serve in 

 functionally associating the two lateral halves of the grey column. 



Each lateral half of the grey 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, them- 

 selves spoken of as columns of grey substance. The dorsal horn, or columna poste- 

 rior, extends posteriorly and somewhat laterally towards the surface of the cord along 

 the line of the postero-lateral sulcus. It is composed of an apex (caput) and a neck 

 (cervix). 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 designated as gelatinous substance of Rolando, to distinguish it from a 

 similar appearance immediately about the central canal (central gelatinous substance). 

 The apex of the dorsal horn is widest in the regions of the enlargements 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 sub- 

 stance 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 tho- 

 racic or first lumbar segment. About the ventral periphery of the nucleus dorsalis 

 are scattered- nerve-cells of the same type as contained in it. These cells compose 

 Stilling's nucleus. 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, or columna anterior, of each lateral half of the grey figure is 

 directed ventrally towards the surface of the spinal cord, pointing towards the antero- 

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

 tral root axones, and these axones make their emergence from the spinal cord along 



