DEVELOPMENT OF MIND IN ANIMALS I29 



vertebrates appear, the brain is definitely divided into three 

 primary regions, fore-, mid-, and hindbrain. 



The hindbrain, the main portion of which is the medulla 

 oblongata, appears as a rather swollen section of the spinal 

 cord. The medulla shows some, but not great, variation 

 throughout the vertebrate series and, besides being a sort of 

 motor nerve clearing house, has partial control over the 

 functioning of internal organs. At the front part of the 

 hindbrain is a cabbage -shaped structure, the cerebellum, of- 

 ten called the "tree of life." The ear nerves associated with 

 the sense of balance and of position register here, and so this 

 part of the hindbrain has control over body position and 

 muscular coordination. This part of the brain has had a var- 

 ied, up-and-down history in evolution. It is large in those 

 vertebrates which need a keen sense of position, such as fish 

 and birds; smaller and less developed in others, such as the 

 frog. 



The midbrain is primarily associated with sight. In prim- 

 itive vertebrates the optic nerves, although they enter the 

 brain somewhat more to the forward, pass back to small 

 swellings on the midbrain called optic lobes. Evolution fi- 

 nally by-passed this region; and in higher vertebrates, in- 

 cluding man, most of the nerve fibers from the eye are sent 

 directly to the cerebral hemispheres. Here the optic lobes 

 are replaced by four small bodies, the "four twins" or the 

 corpora quadrigemina. Even with these four bodies nature 

 has varied her pattern, since in the highest vertebrates hear- 

 ing is added to the function of the four twins. On the whole 

 the midbrain has undergone the least evolutionary change 

 throughout the history of the vertebrates. 



It is the f orebrain, chiefly, that has undergone the greatest 

 increase in size and specialization. This part of the brain in 

 lower vertebrates is actually and relatively quite small and 

 has mainly to do with the sense of smell. Olfactory lobes, 

 small swellings on the front of the forebrain, are about the 

 only structures present in the earliest vertebrates. De-em- 



