I.] THE FROG. 141 



e. The pia mater; obvious as a darkly stained in- 

 vestment for the whole. It dips in, especially 

 at the fissures, to form a supporting framework 

 for the cord. Prolongations of it into the 

 substance of the cord may be readily seen, in 

 the vicinity of the ventral fissure. 



ii. Examine under a high power. 



a. The white matter; chiefly composed of medul- 

 lated fibres, cut across and lying within a sup- 

 porting meshwork (neuroglia). Non-medullated 

 fibres may be detected here and there. 



b. The grey matter. 



a. Composed, for the most part, of non-medul- 

 lated fibres lodged in a granular matrix. 



Note the course of the fibres, especially 

 of those which pass from side to side. 



/8. Multipolar nerve-cells. A cluster of these will 

 be seen in each ventral grey cornu, as large 

 irregular cells, deeply staining and each with 

 a round nucleus. Nerve fibres will be seen 

 to enter them at several points. 



y. Ventral nerve root. Examine with care; con- 

 tinuity may often be traced between its indi- 

 vidual fibres and the multipolar nerve cells. 



12. The retina. 



Highly satisfactory preparations of this organ can 

 be obtained as follows. Take perfectly fresh eyes 

 from a frog ; remove a small portion of each, and lay 

 the eyes for an hour in corrosive sublimate, or for 



