CENTRAL RELATIONS OF COMPONENTS. 57 



median part) which corresponds to that of the medial part 

 of the dorso-lateral column of the cord." 



In judging- of the correctness of this assumption we 

 must first determine what is the nature of the movement 

 by which the canalis centralis has expanded to form the 

 fourth ventricle. Remembering that the roof-plate of the 

 nerve tube is membranous in the embryonic condition and 

 is typically so in the adult, any massive structure appear- 

 ing in it being a secondary ingrowth from the sides, it 

 would appear that the membranous roof of the fourth 

 ventricle represents this roof-plate and is represented in 

 the spinal cord only by the floor of the dorsal fissure. 

 The fourth ventricle is formed, then, by the dorsal and 

 lateral expansion of the canalis centralis so that dorso- 

 median structures of the cord become dorso-lateral struc- 

 tures of the oblongata. This is clearly shown by the 

 course of the spinal V tract, which is unquestionably the 

 cranial continuation of the dorsal horn (or at least of its 

 general cutaneous portion). It is, then, obviously im- 

 possible to homologize structures lying in the floor of the 

 fourth ventricle with those lying dorsally of the canalis 

 spinalis, and this is what Goronowitsch and Haller 

 attempt. The lobus vagi is developed in the floor of the 

 fourth ventricle mesially and hence morphologically 

 ventrally of the spinal V tract and the corresponding 

 position in the cord must lie ventrally of the dorsal cornu. 

 The spinal representative of the communis system of the 

 head (visceral sensory), if such a component exists in the 

 trunk, should have its terminal centre in the dorsal part of 

 the intermediate zone, while the viscero-motor centre 

 should occupy the ventral part of that zone. 



This intermediate zone in higher animals contains the 

 lateral cornu, Clarke's column and other structures now 



