864 



THE SUBSTANCE OF ROLANDO. [BOOK in. 



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 subse- 

 quent growth takes up a position at the end of the posterior horn. 

 Hence, though it never apparently contains any cavity, the sub- 

 stance of Rolando may be regarded as an isolated portion of the 

 walls of the medullary canal, which has undergone 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 conus medullaris, the central canal 

 widens out dorsally, and in section (Fig. 97, A} presents on each 

 side a bay x, stretching out towards the position of the posterior 

 horn. At this region of the cord, though both white and grey 

 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 



A B C 



FIG. 97. DIAGRAM TO ILLUSTRATE THE NATURE OF THE SUBSTANCE OF ROLANDO. 



The figures are purely diagrammatic and are not drawn to the same scale. In 

 all three figures the grey 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. epithe- 



lium lining the medullary canal, .r. lateral expansion of the canal. 



B. transverse section of the spinal cord of the calf in the lower thoracic region. 



r. substance of Eolando. c. central canal. 



G. transverse section through mid thoracic region of cord in man. 



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. 97, -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 



