346 AMPHIBIA xii. 24 



The brain (Figs. 203 and 204) resembles that of Dipnoi very strik- 

 ingly. The prosencephalon is based on an inverted plan (p. 211); the 

 large evaginated cerebral hemispheres therefore have a thick nervous 

 roof as well as floor. In the frog there is only a short unpaired region 

 of the forebrain (diencephalon) but this is longer in urodeles. The 

 walls of each hemisphere may be divided into a dorsal pallium, medial 

 ventral septum, and latero-ventral striatum (Fig. 205). The cell bodies 

 lie around the ventricle in all parts of the hemisphere and there are 

 several layers of them. The cells are pyramidal in shape and the con- 

 nexions are made in the outer 'white' matter. 



Nearly all parts of the hemisphere are reached by olfactory tract 

 fibres, the axons of the mitral cells of the olfactory bulb (Fig. 205). 

 In the frog there are regions at the hind end of the hemispheres that 

 receive forwardly directed fibres, some probably connected with tactile 

 and others certainly with optic impulses. There is therefore some 

 opportunity for the hemispheres to act as correlating centres, but we 

 have little information as to the functions performed in them. Their 

 backward projections are made by means of two large tracts, the 

 lateral and medial forebrain bundles, but these reach only to the 

 thalamus, hypothalamus, and midbrain, not back to the cord. Elec- 

 trical stimulation of the forebrain does not produce movements of the 

 animal; presumably such a crude method, though it may excite a 

 few neurons, cannot imitate the more subtle patterns in which they 

 are normally active. 



Removal of the cerebral hemispheres is said to have little influence 

 on the normal feeding and other reactions of the frog. After this 

 operation the animals are said to be more sluggish, to show less 

 'spontaneity', and to learn less well. If the latter is true it shows a 

 considerable advance in the functioning of the hemispheres over the 

 stage reached in fishes, whose learning can certainly take place in 

 other parts of the brain, and is apparently little affected by removal 

 of the forebrain (p. 210). 



Some indication of the function of the cerebral hemispheres is given 

 by the fact that by placing electrodes connected with a suitable am- 

 plifier upon them, rhythmical changes of potential can be recorded 

 (Fig. 206). These are most marked in the olfactory bulb and probably 

 propagate backwards along the hemisphere. The rhythms continue 

 even in a brain that has been removed from the head. They are there- 

 fore a sign of some intrinsic activity of the brain, rather than of re- 

 sponse to peripheral stimulation. 



The diencephalon is interesting chiefly for the considerable number 



