THE STRUCTURE OF THE SPINAL CORD. 535 



sisting of a somewhat peculiar neuroglia traversed by fibres of the posterior 

 root, and containing a large number of cells which, for the most part small, 

 the cell-bodies being- small relatively to the nuclei, are not all alike, some 

 being probably nervous and others not. It takes origin from the cells form- 

 ing the immediate walls of the embryonic medullary canal. In the embryo 

 this canal is relatively wide, though compressed from side to side, and in 

 transverse sections of the medullary tube appears at a certain stage as a 

 narrow oval slit placed vertically and reaching almost from the dorsal to the 

 ventral surface. The dorsal part of this long slit is later on closed up by 

 the coming together of the walls and the obliteration of the greater part of 

 the cavity, leaving the ventral part to form a circular canal, which by the 

 development of the anterior columns assumes the central position. During 

 this closure of the dorsal part of the canal a mass of the cells lining the 

 canal is cut from the rest on each side, and during the subsequent growth 

 takes up a position at the end of the posterior horn. Hence, though it never 

 apparently contains any cavity, the substance of Rolando may be regarded 

 as an isolated portion of the walls of the medullary canal, which has under- 

 gone a development somewhat different from that of the portion which 

 remains as the lining of the central canal. Traces of this origin may be 

 seen even in the adult. Thus, in the lower end of the cord, in what we shall 

 speak of presently as the conns medullaris, the central canal widens out dor- 

 sally, and in section (Fig. 120, A) presents on each side a bay x stretching 



FIG. 120. 



Diagram to illustrate the Nature of the Substance of Rolando. The figures are purely dia- 

 grammatic and are not drawn to the same scale. In all three figures the gray matter is shaded 

 with fine lines and the white matter with dots. A, transverse section of the lower end of the 

 conus medullaris in man ; e, epithelium lining the medullary canal ; x, lateral expansion of the 

 canal; B, transverse section of the spinal cord of the calf in the lower thoracic region; r, sub- 

 stance of Rolando : c, central canal ; C, transverse section through mid-thoracic region of cord in 

 man. 



out toward the position of the posterior horn. At this region of the cord, 

 though both white and gray matter are developed on the ventral surface, the 

 posterior columns do not meet on the dorsal surface, but leave the central 

 canal covered only by tissue which perhaps may be called neuroglia, but is 

 of peculiar nature and origin. In the calf, in a part of the dorsal region the 

 substance of Rolando is not confined to the tip of the posterior horn, but is 

 continued to meet its fellow in the middle line. (Fig. 120, B.} If we imagine 

 the dorsal portion of the canal of A to be cut off from the ventral portion, 

 its cavity to be obliterated, and the lining epithelium with some of the sur- 

 rounding elements to undergo a special development, the condition in B is 

 reached by the growth of the posterior columns. From B the transition to 

 the normal state of things as in Fig. 120, c, is a very slight one. The extreme 

 dorsal tip of the horn, being of a more open texture than the substance of 

 Rolando, is sometimes called the zona spongiosa. 



479. The grouping of the nerve-cells. The nerve-cells, at all events the 

 cells which are large enough to to be easily and without doubt recognized to 



