456 THE ORGANS 



between it and the periphery, is a zone composed of fine longitudinal meduUated 

 fibres rather sparsely arranged and therefore staining more lightly with the Wei- 

 gert method. This is the zona terminalis or zone of Lissauer. It belongs ob- 

 viously to the white matter of the cord (see page 458). The portion of the gray 

 matter connecting the dorsal and ventral horns may be termed the intermediate 

 or middle gray. Note the well-defined groups of large nerve cells in the anterior 

 horns and the fibres passing out from the anterior horns to the surface of the cord, 

 ventral {anterior) nerve roots. (Figs. 319 and 320.) 



White jVIatter. — Note the general appearance of the white matter and the 

 disposition of the supporting strands of neuroglia tissue (light in the Weigert, 

 usually darker in other stains). The neuroglia is seen to form a fairly thick 

 layer just beneath the pia mater from which trabeculae pass in among the fibres, 

 the broadest strand forming the posterior median septum. If the section has 

 been cut through a dorsal {posterior) nerve root, a strong bundle of dorsal root 

 fibres can be seen entering the white matter of the cord along the dorsal and mesial 

 side of the posterior horn. Just ventral to the anterior gray commissure is a 

 bundle of transversely-running medullated fibres — the ventral white commissure. 

 (Fig. 320.) It is composed of the axones of various heteromeric column cells 

 and of decussating terminals of various fibres. In the dorsal part of the dorsal 

 gray commissure are also a few fine transversely running medullated nerve 

 fibres — the dorsal white commissure. It consists of collaterals of fibres in dorsal 

 funiculi and axones of heteromeric column cells. 



Cell Groupings. — The positions and groupings of the various cell bodies 

 should now be studied. (Nissl, Haematoxylin-Eosin, Cajal, Figs. 319 and 320.) 

 (For convenience, their general arrangement throughout the cord is here 

 given; also the course of their axones, though this is usually only seen in Golgi 

 preparations.) 



(A) Cells of the Dorsal Horn. — (a) Marginal cells arranged tangentially 

 to the border of the gelatinous substance of Rolando, (b) Cells in the gelatinous 

 substance of Rolando, arranged radially. The Golgi method indicates that the 

 axones of (a) and (b) principally enter the adjoining lateral column, (c) Large 

 stellate cells in the apex of the caput, most of the axones of which go to the lateral 

 columns, but some cross in the ventral white commissure, (d) A central group 

 in the central part of the dorsal cornu, some of the axones of which may cross in 

 the ventral commissure, (e) Basal cells in the base of the horn and in the pro- 

 cessus reticularis, the axones of which usually go to the lateral column but may 

 cross, (f) Dorsal (thoracic) nucleus or cells of Clarke's column in the mesial part 

 of the base of the dorsal horn. These are tautomeric cells, the axones of which 

 from the dorsal spino-cerebellar tract (see page 469). Clarke's column is mainly 

 confined to the dorsal or thoracic cord, but is also present in the first lumbar 

 segment. 



(B) Cells of the Intermediate Gray. — (a) Middle nucleus whose cells 

 may send their axones across in the ventral commissure or uncrossed to the lateral 

 column; (b) various small cells including the accessory nucleus near Clarke's 

 column; (c) intermedio-lateral group which in parts of the cord forms a projection 

 of the gray known as the lateral horn. These are probably root cells whose 

 axones pass as preganglionic fibres into the sympathetic system. This nucleus 



