142 THE ANATOMY OF THE HORSE. 



no actual existence, but is sometimes described as extending along the 

 line of emergence of the inferior roots. 



These fissures will be better seen in a transverse section of the cord. 

 This should be made with a sharp scalpel, so as to leave a clean-cut 

 surface. On examining this surface, the superior median fissure will be 

 seen to extend inwards to near the centre of the cord, while the inferior 

 median fissure, which is wider but not so deep, also extends towards 

 the centre of the cord. The superior and inferior median fissures do 

 not quite meet, being separated by a bridge of tissue connecting the 

 right and left halves of the cord. This bridge of tissue is made up of 

 the grey and white commissures of the cord. The grey commissure stretches 

 across the bottom of the superior median fissure, and in its centre there 

 will be seen a dot-like mark, which is the section of the central canal of 

 the spinal cord. This canal extends throughout the whole length of the 

 spinal cord ; and where the cord joins the brain, the canal is continued 

 into the medulla oblongata, in which it opens into the 4th ventricle. 

 The white commissure forms a thinner stratum than the preceding, 

 beneath which it stretches at the top of the inferior median fissure. 



It will be observed that in each half of the cord there are two kinds 

 of nerve tissue, distinguished by a difference in colour. 



1. There is the grey matter, which lies in the interior, and has a 

 crescentic form. The convex side of each crescent is turned inwards, 

 and the right and left crescents are connected by the grey commissure. 

 The extremities of the crescent are termed its horns. The superior horn 

 is acute, and is prolonged to the supero-lateral fissure by a single 

 bundle of fibres belonging to a superior nerve-root. The inferior horn 

 is rounded, and lies some distance beneath the surface of the cord. 

 From it several bundles of fibres pass to form an inferior nerve-root. 

 The grey matter of the cord contains nerve cells, medullated and non- 

 medullated nerve fibres, and delicate nerve fibrillse. The nerve cells 

 are mostly of the multipolar variety, and are most numerous in the 

 inferior horn. The connective-tissue of the cord, both here and in the 

 white matter, is a delicate substance termed neuroglia. 



2. The white matter in each half of the cord surrounds the crescent, 

 and it is divided into three columns by the crescent and the nerve 

 bundles passing from the horns. The superior column lies between the 

 superior median fissure and the upper half of the crescent. The inferior 

 column is included between the inferior median fissure and the lower half 

 of the crescent. The lateral column lies in the concavity of the crescent, 

 its limits being marked at the surface of the cord by the supero- 

 lateral fissure and the line of emergence of the inferior nerve-roots. 

 The white matter of the cord is composed, besides neuroglia, of medul- 

 lated nerve fibres having for the most part a longitudinal direction. 



{For the muscles of this chapter, see the table at page 336.) 



