Mammalia: Nervous System 717 



primary olfactory centers in front are connected with more posterior 

 sensory centers. The primary sensory centers of the rear parts of the 

 brain are connected with one another by association-tracts. Numer- 

 ous correlation-centers establish a great variety of relations between 

 primary centers. Out of this maze of centers and tracts in the palae- 

 encephalic part of the brain emerge great projection- tracts (Fig. 525) 

 which pass up into the cerebral cortex. Centers in the thalami have 

 been described as being especially important "way stations" on the 

 road to the cortex. Within the cortex a complex system of association- 

 tracts (Fig. 526) connects one region of the cortex with another. Fi- 

 nally, from the cortex come out tracts of efferent nerve-fibers trans- 

 mitting impulses which are relayed eventually to the motor and 

 glandular effectors. 



It is in relation to these efferent tracts which pass downward and 

 backward from the cerebral cortex that the cerebellum plays its im- 

 portant role. The connections of the cerebellum with the lower parts 

 of the brain are very similar to those of the cerebral cortex. Tracts of 

 fibers from all the sensory centers pass up into the cerebellar cortex, 

 and descending efferent tracts connect with the numerous motor 

 centers of the lower part of the brain. Also, and highly important, 

 there are extensive connections between cerebral and cerebellar 

 cortices. 



The supreme governing body seated within the brain is the cere- 

 bral cortex. The cerebellar cortex is the executive department. The 

 cerebral cortex "decides" what is to be done and when and how. The 

 decision is based not merely on the information received at the moment 

 but also on past experience. Sensory impressions are somehow — no 

 one knows how — retained by cortical cells. The cortex becomes a 

 library stored with records of the history of the animal's life. If the 

 animal is one that can read a printed language, its cortical content 

 becomes expanded to include the experiences of other lives and knowl- 

 edge of things and events indefinitely remote in space and time. Action 

 resulting from a stimulus may not be immediate. Time may be taken 

 to "think it over," and in the course of the thinking the stored experi- 

 ence and knowledge are, or should be, taken into account. If the 

 decision reached calls for muscular activity, the cerebral cortex (so 

 far as we are aware) concerns itself with the action only in a large and 

 general way, not with the details of the action of individual muscles. 

 All of that is left to the "executive department." As the general orders 

 go down the descending tracts from the cerebral cortex, "information" 

 as to what is to be done is sent also to the cerebellum. The specified 

 acts may involve the harmonious cooperation of scores of small 

 muscles. All of this detailed work of coordination is automatically 



