CHAP, ii.] THE BRAIN. 939 



has the appearance of a strand broad behind but thinning away 

 in front, while the external posterior column, also broadening as it 

 advances forwards, seems to be wedged in between the median 

 posterior column on its median edge and the restiform body on its 

 lateral edge ; hence the former is here called the fasciculus (or 

 faniculus) gracilis (m. p.), and the latter the fasciculus (or funi- 

 culus) cuneatm (e. p.). Further forward both columns seem to 

 merge with each other and with fibres which curve round to form 

 part of the restiform body ; the relations however of these two 

 columns to each other and to the other parts of the bulb, as well 

 as the nature of the other several changes by which the cord is 

 transformed into the bulb, are disclosed by transverse vertical 

 (dorso-ventral) sections, to the study of which we must now turn. 



A section (Fig. 109, 1) taking at the hind margin of the 

 decussation, at which level the first cervical nerve takes origin, 

 when compared with a section of the cord at the level of the 

 second cervical nerve (cf. Fig. 104, C 2 ), shews that certain changes 

 are already taking place in the grey matter. The anterior horns 

 are not much altered, but the posterior horns are, as it were, 

 pushed out laterally and dorsally so that the posterior columns, 

 which as yet retain their previous great depth, become very 

 much broader than they are lower down, encroaching, so to 

 speak, on the lateral columns. At the same time the substance 

 of Rolando (s. g.}, forming the head or caput of the horn, has 

 enlarged into a more or less globular form, and lies near the 

 surface of the cord though separated from it by a compact tract 

 of longitudinal fibres (V. a.), which as we shall see, belongs to the 

 fifth cranial nerve. A considerable development of the reticular 

 formation (/. ret.) at the side of the grey matter ventral to the 

 posterior horn has also taken place, and this with the shifting 

 of the position of the posterior horn has driven the lateral horn 

 (l.h.) nearer to the anterior horn. From this lateral horn a root 

 of the eleventh spinal accessory cranial nerve (xi), may be seen 

 taking origin. Further, a great increase of grey matter round the 

 central canal may also be observed. 



These changes, however, are of degree only ; what seems to 

 be an absolutely new feature is the presence of bundles of fibres 

 (Py. dec.), which starting from the anterior column of one side 

 cross over to and are apparently lost in the grey matter of the 

 neck of the anterior horn of the other side ; in so crossing the 

 fibres push aside the bottom of the anterior fissure. When the 

 course of these fibres is investigated, either by simple microscopic 

 observation, or still better by the method of degeneration, it is 

 found that they may be traced from the anterior column of one 

 side, across the anterior commissure, through the neck of the 

 anterior horn to the lateral column of the opposite side, and to 

 that part of the lateral column which we have previously de- 

 scribed as the crossed pyramidal tract. 



