CHAP, i.] THE SPINAL CORD. 859 



lat. col] between the posterior root and the most external bundle 

 of the anterior root, and into an anterior column (Fig. 96, ant. 

 col.) between the anterior fissure and the most external bundle 

 of the anterior root. The part traversed by the bundles of the 

 anterior root, as they make for the anterior horn, accordingly 

 belongs to the anterior column ; but some writers speak of the 

 anterior column as lying between the anterior fissure and the 

 nearest bundle of the anterior root, thus making the region of the 

 anterior root belong to neither anterior nor lateral column. And 

 indeed the distinction between the anterior and the lateral 

 column is to a great extent an artificial distinction. 



563. The ' white matter ' consists exclusively of medullated 

 fibres supported partly by connective tissue and partly by a peculiar 

 tissue known as neuroglia, of which we shall presently speak. The 

 fibres are of various sizes, but many of them are large, and in all 

 of them the medulla is conspicuous. They run for the most part 

 longitudinally, so that in transverse sections of the cord nearly the 

 whole of the white matter appears under the microscope to be 

 composed of minute circles, the transverse sections of the lon- 

 gitudinally disposed fibres, imbedded in the supporting structures. 

 The 'grey matter' also contains medullated fibres, but these are 

 for the most part exceedingly fine fibres possessing a medulla 

 which appears to differ from that of an ordinary nerve-fibre, since 

 it does not stain readily with osmic acid, but is rendered visible by 

 special modes of preparation such as that known as Weigert's. 

 Hence these fine fibres are not apparent in ordinary carmine or 

 other specimens, and indeed their presence was for a long time 

 overlooked. Besides these fine medullated fibres, if we may call 

 them such, the grey matter contains, what the white matter does 

 not, nerve-cells with branching processes, naked axis-cylinders, and 

 delicate filaments arising from the division of axis-cylinders or 

 from the branching of nerve-cells, all these various structures 

 being imbedded in neuroglia. Owing to the relative abundance 

 of the white refractive medulla, the white matter possesses in 

 fresh specimens a characteristic opaque white colour; hence the 

 name. The grey matter from the relative scantiness of medulla 

 has no such opaque whiteness, is much more translucent, and 

 in fresh specimens has a grey or rather pinky grey colour, the 

 reddish tint being due to the presence partly of pigment and 

 partly of blood, for the blood vessels are much more abundant in 

 the grey matter than in the white. 



The pia mater which closely invests the cord all round consists 

 of connective tissue, fairly rich in elastic elements and abun- 

 dantly supplied with blood vessels ; it is indeed essentially a 

 vascular membrane and furnishes the nervous elements of the 

 cord with their chief supply of blood. It sends in at intervals 

 partitions or septa of the same nature as itself radiating towards 

 the central grey matter. The narrow posterior fissure is com- 



