'HISTOLOGY OF THE CENTRAL NERVOUS SYSTEM OF THE WEDDELL SEAL. 153 



grey matter into two main groups, an anterior and a posterior. Just beneath the 

 superficial white matter several well-defined groups of nerve-cells may be seen (n.), and 

 posteriorly a group is also to be distinguished. Many small nerve-cells are scattered 

 throughout the masses of grey matter. 



VI. THE HISTOLOGY OF THE CONVOLUTIONS OF THE PRECENTRAL GYRUS. 



(Text-fig. 5.) 



The cortex of the motor area presents a fairly typical structure, except that large 

 multipolar nerve-cells occur at a relatively deep level, forming a deeply staining layer 



FIG. 5. Semidiagrammatie vertical section of a part of the motor cortex cerebri (precentral convolution), x 6. 



1. Plexiform layer. 



2. Layer of small nerve-cells (granules). 



3. Layer of large flask-shaped nerve-cells, the axons passing centrally, dendrons peripherally. 



4. Layer of large pyramidal cells (comp. to Bett's cells of human motor cortex). 



5. Deep layer of large pyramids, lying next to the white centre : there is much dense neuroglia in this layer. 



lying next the fibres of the white centre ; in all, about four layers of nerve-cells may 

 be distinguished. The following layers are quite distinct : 



(a) A superficial layer formed of interwoven fibres (plexiform layer). 



(b) A second layer of small nerve cells, the axons of which are not very obvious. 



(c) A third layer of large flask-shaped cells not unlike the Purkinje cells of the 



cerebellum, with axons passing centrally and dendrons peripherally. 



(d) A fourth layer of rather large pyramidal cells, the axons passing centrally. 



(e) A fifth layer of large multipolar cells, the axons passing in many directions ; 



this layer contains much deeply staining neuroglia. 



(ROT. SOC. EDIN. TRANS., VOL. XLVIII., 861.) 



