STRUCTURAL ARRANGEMENTS OF CEREBRUM 471 



that of the centres forming the brain stem or the olfactory lobe itself 

 in that it provides for a very rich association of impulses between all its 

 parts. The fibres entering the cortex break up into a fine meshwork 

 of fibres which run tangentially to the surface and come in contact 

 with innumerable dendrites of nerve-cells situated at some little dis- 

 tance below the surface (Fig. 207). We have here the first germ of 

 an apparatus in which the nerve-paths can be determined by education, 

 i.e. in consequence of past inhibitions by pain, rather than by the 

 limits set by heredity. In the amphibian brain, and still more in the 

 brain of the reptile, the cerebral cortex extends over the whole of the 

 roof of the cerebral hemispheres, though even here a very large pro- 

 portion of it is devoted to the association of olfactory impulses. The 

 importance of these olfactory association fibres is well shown in the 



FIG. 208. Schematic section through brain of lizard showing the chief 

 nerve-tracts. (After EDINGEB.) 



figure (Fig. 208) of a diagrammatic section through a lizard's brain. 

 Above the reptiles there is a divergence in the course of development. 

 The wider reactive powers of birds are based chiefly on an enormous 

 development of the corpus striatum, whereas in mammals the corpus 

 striatum remains relatively small and the chief development occurs 

 in the roof of the cerebral hemispheres, the so-called pallium or mantle. 

 With the increased entry of fibres from the optic thalamus into the 

 cerebral hemispheres, carrying impulses from the eyes, ears, and all 

 the other sense-organs of the body, the olfactory part of the brain 

 diminishes in importance, and in the higher mammal and man is alto- 

 gether overshadowed by the newly formed part of the pallium. On 

 this account those parts of the cerebral hemispheres in special connec- 

 tion with the olfactory sense-organs are often spoken of as the archi- 

 pallium, in distinction to all the rest of the more newly formed brain 

 substance, known as the neopallium. 



In man the cerebral hemispheres form a great ovoid mass exceeding 



