COORDINATION 203 



The three-vesicle brain now becomes transformed into one of 

 five vesicles by a hollow outpocketing from the anterior end of the 

 fore-brain and a dorsal outpocketing from the hind-brain. In the 

 lowest forms the brain throughout life consists essentially of these 

 divisions, known as telencephalon, diencephalon, mid-brain, 

 cerebellum, and medulla oblongata; the latter merging into 

 the spinal cord. Usually, however, the telencephalon gives rise to 

 a pair of cerebral hemispheres which are destined gradually to 

 overshadow in size and significance all the other parts of the brain. 

 Then the development from the telencephalon or its derivatives, 

 the cerebral hemispheres, of a pair of olfactory lobes completes 

 the establishment of the chief brain chambers. 



The further changes which transform the more or less linear 

 series of vesicles into the increasingly complex and compact brain 

 of higher forms are due to bendings, or flexures, and to unequal 

 thickenings and outgrowths of the chamber walls. For instance, 

 the upper and lower surfaces of the diencephalon give rise to the 

 pineal body and the infundibulum respectively, while from 

 similar regions of the mid-brain are developed the optic lobes 

 and crura cerebri. Hand in hand with these changes the pri- 

 mary cavities of the chambers undergo a gradual restriction, but 

 throughout all there persists at least a remnant of the original 

 tubular cavity which is continuous with that of the spinal cord. 

 (Figs. 109, 141.) 



The cerebrum, or cerebral hemispheres, is considerably the 

 largest and most important part of the human brain since it is 

 the center of perceiving, thinking, voluntary motion, and even 

 consciousness — the seat of the higher mental life in general. These 

 primary activities are performed by neurons, the cell bodies being 

 located in the cortex, the outer layer of gray matter, while the 

 nerve fibers from the neurons extend deeper to form the inner 

 white matter. These fibers transmit nerve impulses to and from 

 the cell bodies in the cortex. 



Next in importance is the cerebellum which, of course, also con- 

 sists of neurons. Their functions are subsidiary to those of the 

 cerebrum since initiative resides in the cerebrum, but messages 

 from this director are coordinated by the cerebellum on their way 

 to various parts of the body. Thus, one may consciously extend 

 an arm, but the various compensating movements of other parts 

 of the body that are necessary to maintain equilibrium are at- 



