120 



THE VERTEBRATE ANIMAL: IRRITABILITY 



on. 



o.c. 



made in the diencephalon, and pain sensations are received there. 

 Other centers are in the nature of important relay stations in the 

 compounding of reflexes. 



Among the lower vertebrates, the most important parts of the 

 telencephalon are the centers for correlation of impulses trans- 

 mitted from the olfactory organs. The olfactory centers in mam- 

 mals occupy the same relative position, but are overshadowed by 

 the very great growth of the roof of the telencephalon to form the 

 cerebral hemispheres. In the cerebral hemispheres, as in the 



cerebellum, the neurones 

 that make up the gray 

 matter are found in a con- 

 tinuous superficial layer 

 known as the cerebral 

 cortex. Although the cor- 

 tex is continuous, certain 

 areas are known to be con- 

 cerned with special func- 

 tions. Impulses producing 

 movements of the voluntary 

 muscles are sent out to 

 opposite sides of the body 

 from the motor centers of 

 the cortex; that is, if 

 these particular areas of 

 the cortex are destroyed 

 the animal is unable to 

 use the voluntary muscles on the opposite side of the body. In 

 man, the regions that control movements of the principal parts of 

 the body, from the toes to the face muscles, are known. Another 

 major division of the cortex is concerned with sensory func- 

 tions, and contains the sensory centers, to which impulses are 

 transmitted from visual, auditory, and olfactory organs, as well as 

 from receptors of pressure, pain, temperature, and taste. These 

 areas have been almost completely mapped out for the human 

 cortex. Surrounding and connecting the motor and sensory centers 

 which occupy a relatively small part of the cortex, are the asso- 

 ciation centers. These regions are filled with adjustor neurones 

 which are responsible for the complicated pathways of learning 

 processes and which are involved in memory. 



Fig. 65. — Diagram of the optic chiasma in 



higher vertebrates. Notice that only the 



medial nerv'e fibers cross. 



o.c, optic chiasma; o.n., optic nerve; o.t., optic 

 tract which ends in the optic lobe; r, retina. 



