348 THE MAINTENANCE OF THE INDIVIDUAL 



from which the olfactory nerves pass directly to the nostrils, thus 

 receiving stimuli which are interpreted in the brain as odors. 



The cerebral hemispheres contain cavities known as the first and sec- 

 ond ventricles, which are continuous posteriorly with the third ventricle, 

 found in the 'twixt-brain or diencephalon. Dorsally and laterally the 

 cerebral walls are known as the pallium, that furnishes a foundation 

 on which the outer layer, or coriex is developed. In the higher verte- 

 brates a connecting bridge of white fibers, called the corpus callosum, 

 unites the two cerebral hemispheres. The higher the mammal is in 

 the scale of life the more convoluted the cortical surfaces of the hemi- 

 spheres become, and the more the cerebrum weighs in proportion to 

 the rest of the organ. A rabbit's cerebnun composes slightly more 

 than half of the mass of the entire brain, while in man it exceeds four 

 fifths of the total weight. 



The sui)erficial cortical layer of the cerebrum forms a mass made 

 up of numberless nerve cells interwoven into an intercommunicating 

 network. The axons of some of these neurons pass over the bridge 

 of the corpus callosum from one side to the other while other axons 

 extend downward in great bands as far as the cerebellum and to other 

 more posterior centers. It has been demonstrated that this portion 

 of the brain is the seat of consciousness and the controlling center of all 

 our higher mental life. As the cerebral functions increase, the instinc- 

 tive reflexes retire further into the background. Herein lies the chief 

 difference between the so-called lower animals and the higher ones. 

 The former are chiefly at the mercy of their hereditary limitations and 

 their environment, while the latter have risen sufficiently above their 

 environmental conditions to begin at least to become "the captains of 

 their souls." A series of experimental operations on a dog, in which 

 the entire cerebrum was finally removed, illustrates the importance of 

 this part of the brain to the higher animals. The dog in question 

 apparently became an idiot, unable to associate experiences or to 

 learn. It had no ability to differentiate between solid objects in its 

 path and patches of sunlight on the floor, which could in no way 

 hinder its progress. 



The 'Twixt-Brain or Diencephalon. This region is compara- 

 tively inconspicuous, but very essential to the biologist, since the 

 ventral floor of the 'twixt-brain gives rise to an outgrowth called the 

 infundibulum, which fuses with a dorsal outgrowth from the roof of 

 the mouth to form the pituitary gland (hypophysis), the "generalissimo" 

 of the ductless glands. Possibly because of its importance it has 



