NERVOUS SYSTEM. VERTEBRATA. 143 



explanation of the large size of the neopalliuin in the 

 Monotremes (and in Tachyglossus more especially) cannot 

 be offered at present. Professor C. J. Martin has shown that 

 stimulation of the cerebral cortex of Platypus in the situa- 

 tions shown in figures 34 and 37 gives rise to definite 

 muscular actions: thus stimulation of the " arm-area" 

 (1) produces a digging movement in the opposite fore- 

 limb, of areas (2) and (3) produces movements of the 

 eyelids, and of (4) retraction of the head (Journ. of Phys. 

 vol. xxiii. 1898, p. 383). This shows that physiologically 

 the cerebral cortex of Platypus reacts in a manner analogous 

 to that of lowly placental mammals. 



In striking contrast to the relatively large size of the 

 cerebral cortex, is the smallness of the efferent tracts which 

 pass from it to other regions of the central nervous system. 

 The pes pedunculi and the pyramidal tract are so insigni- 

 ficant, that they give rise to no projection upon the surface 

 of the brain. The pyramidal decussation is very diffuse. 



With the exception of the hippocampal and rhinal 

 fissures, which subdivide the pallium into its three funda- 

 mental constituent parts, the hemisphere of the Platypus is 

 devoid of true fissures or sulci. The general smoothness 

 of the neopalliuin is, however, disturbed by depressions for 

 the bony capsules of the floccular lobes of the cerebellum 

 (compare figs. 34 & 35) and by a series of vascular furrows. 

 The most noteworthy of the latter consist of -a series of 

 large shallow channels produced by very large veins, which 

 converge towards the apex of the hemisphere (fig. 37). 

 The large size of these veins is due to the fact that the 

 longitudinal venous sinus is practically, if not completely, 

 obliterated (because the falx cerebri is bony), and the 

 general cerebral veins enlarge to replace them. The 

 spurious resemblance of the brain of Platypus to the avian 

 type is thus considerably accentuated, because in many 

 birds an analogous arrangement of the cerebral veins is 

 found. 



Several well-marked transverse furrows are produced by 

 the branches of the middle and anterior cerebral arteries. 

 The cerebellum of Ornithorhynchus is a slightly simplified 



