112 HUMAN BIOLOGY 



but to the details of a luminous field as the higher animals 

 are able to do. With the growth of eyes of this type in the 

 early vertebrates came the concomitant development of the 

 optic lobes of the brain, a step that estabHshed these organs 

 as the chief receptor centers of the simpler vertebrates. 



All fishes that possess eyes also have the so-called ear 

 sacs. These simple ears are chiefly concerned with positional 

 relations, equilibrium and the like. But they also have to do 

 with hearing and both functions develop hand in hand in 

 higher forms influencing the growth of the brain in the 

 region of the cerebellum and the medulla oblongata. Thus 

 this third and last kind of distance receptor contributes its 

 share to brain formation. 



In this way the evolutionary growth of the vertebrate 

 brain and with it the head has resulted from a change of its 

 primitive surface receptors to distance receptors whereby 

 highly specialized nasal cavities, eyes, and ears were the 

 external products and an olfactory, a visual, and a positional 

 brain were the internal results. These collectively establish 

 in the vertebrates what has been called the stem of the 

 brain. 



But this stem carries with it more than the three sensory 

 segments just accounted for. Of the additional elements in 

 the brain stem, the chief one is found in the region of the 

 hemispheres. In the lower vertebrates the two lobes at the 

 anterior end of the brain, the so-called hemispheres, are 

 largely concerned with olfaction. This sensory activity, 

 as already intimated, is the first to develop distance recep- 

 tion. It is, therefore, not surprising to find that in verte- 

 brate evolution the hemispheres came to be organs of unusual 

 importance. Not only did distance reception for the olfactory 

 function reside here, but the hemispheres developed as 

 centers which integrated all the sensory activities including 

 the receptive functions of the skin, of the organs of taste, 

 of sight and of hearing. Thus the hemispheres of the higher 

 vertebrates came to represent a field upon which was reflected 

 the sensory activities of the whole body. Moreover, to this 

 field were transferred eventually all those motor centers 

 which we ordinarily associate in ourselves with volitional 

 movements. Thus by a process of accretion the hemispheres 



