35 

 together hit^her and lower levels of the oblor^ata, have come 

 to have their functiors largely usurped ty the develojiiient of 

 more speciali?.ed tracts. Such neurones are, therel'ore, degra- 

 ded to a far lower plane of metabolic activity, and they respond 

 but feebly to those of our stains which depend upon the presence 

 of definite chemical constituents of the protoplasn. 



b, Corrfiissural Heurones. — The commissural neurones are 

 readily distinguishable from the tract-neurones by their small- 

 er si7.e (Fig. ?.,c.n. ) . Ooii.missural neurones have a wide dis- 

 tribution. They are scattered between the tract-neurones in 

 the vicinity of the median raphe, and they also are to be found 

 in all parts of the lateral region as far dorsal as the base 

 of the general cutaneous nucleus. 



The external morphology of a commissural neurone is repre- 

 sented in Fig.3,c.n. The cell-body is relatively small in 

 proportion to the extension of the dendrites and the axone. 

 The form of the cell is usually an elongated oval or an irreg- 

 ular triangle. The dendrites are quite often only two in num- 

 ber, arising from the ends of the cell. The course taken by 

 the dendrites does not seem to be affected in the least by the 

 tracts of fibres of the formatio i-eticularis, since it fre- 

 quently takes them obliquely through a bundle of arcuate fi- 

 bres. The dendrites are stout at their bases. They give off 

 only a few short branches. The principal axis of the dendrite 

 pursues a rather even course for a comparatively long distance. 



