666 THE CEREBRUM 



is finally changed into the complex association organ of the higher 

 animals. As such it is destined eventually to dominate all processes 

 of life, because it gives rise to the psychic products involved in con- 

 sciousness, perception, volition, thought and memory. In the higher 

 forms, therefore, all reactions are referable in last analysis to the 

 psychic processes, because while they may not always be the direct 

 outcome of cerebral activity, the latter unfailingly determines the 

 condition of the body as a whole and hence, also the power of reaction 

 of its constituent tissues and organs. 



The brain of the higher animals, therefore, possesses a distinguish- 

 ing feature in its many areas of nervous tissue which are primarily 

 intended to be adjuncts to the different motor and sensory mechanisms. 

 This statement, however, should not imply that they are all psychic in 

 their function; in fact, they are not so to begin with. These additions, 

 as has been stated, bring about an increase in the mass and weight of 

 the brain. Thus, we find that the human brain weighs about 1400 

 grams in the male and 1240 grams in the female, and is heavier than 

 that of any of the lower forms, excepting the whale and elephant. 

 Even a casual study of the behavior of these three types of animals 

 will show that man is distinctly superior to the other two, and even 

 to those animals which, relatively speaking, possess the same amount 

 of brain tissue. 



The reason for this is not difficult to detect. It lies in the fact that 

 a large part of the human brain is taken up by nervous material which 

 gives rise to those associations which are necessary for reflection, 

 intelligence, and volition. In other words, the human brain possesses 

 the distinguishing feature of being more of a psychic mechanism than 

 that of any other animal. It will be seen, therefore, that while the 

 absolute amount of brain tissue of such animals as the dog, ape and 

 man remains practically the same in all three, the human brain has 

 lost much of that kind of nervous material which is ordinarily set 

 aside for motor action and sensation. Instead, it has acquired certain 

 units for the formation of those associations which add a distinct 

 psychic quality to these fundamental processes. This gradual evolu- 

 tion of the cerebral hemispheres, therefore, accomplishes a shifting 

 of function from lower centers to a higher psychic realm situated in the 

 cortex. In the human brain, this transfer of function is portrayed 

 best by the relationship existing between the cortical or psychic 

 centers for vision and hearing and the corresponding lower "reflex" 

 centers situated in the thalamus and geniculate body. In other words, 

 man's position in the scale of the Animal Kingdom is determined by 

 the gradual subordination of these lower centers of the brain-stem 

 to the more recently formed cerebral hemispheres and especially 

 to their cortical portions. 



This functional metamorphosis displays itself in an increase in the 

 complexity of the brain rather than in an increase in its weight. 

 Thus, we find that the rabbit's brain presents a very smooth surface, 



