194 



THE GROWTH OF THE BRAIN. 



of this separation and the significance attached to it 

 have come mainly from the observations of His. 1 The 

 following diagram will serve to illustrate the principal 

 points. 



This diagram represents a cross-section of the deve- 

 loping spinal cord. Within is the cavity forming the 

 central canal. The surrounding walls exhibit an inden- 

 tation on either side, which roughly divides each side 

 into two portions, a dorsal and a ventral. The two 

 dorsal portions are the dorsal plates (Fliigelplatten-His) 



and the two ventral por- 

 tions the ventral plates 

 (Grundplatten-His) of this 

 tube. The ventral plates 

 contain all the nerve cells 

 whose neurons grow out of 

 the central nervous system. 

 The dorsal plates contain 

 no such nerve cells ; they 

 receive, however, the fibres 



which grow in from the 

 FIG. 34. Cross-section in the ,, r . , , 



cervical region of a foetal cells forming the spinal 

 human spinal cord, at the ganglia, and in both plates 

 sixth week, X 50 diameters. 

 (Kolliker.) c., central canal ; 

 a.a., groove separating the two 

 plates ; d.p. t dorsal plate ; v.p.^ 

 ventral plate, in which alone 



HB&w^ v ' r ' 



are found the central cells 

 whose neurons are con- 

 fined within the limits of 

 the medullary tube. The 

 groove which separates 



are developed nerve cells pro 



ducing neurons that leave the 



central system ; d.r., dorsal ,1 



nerve root ; v .r., ventral root. these two important por- 

 tions of the wall can be 



followed from one end of the nervous system to the 

 other. On tracing this dividing groove through its 

 entire length, it is found that the relations of the dorsal 



1 His, Abhandl. d. Math. Phys. Cl. d. Kbnigl. Sachs. Gesell- 

 schaft der Wissenschaften, 1889. 



