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THE VERTEBRATE ANIMAL: IRRITABILITY 



known as centers, which are completely surrounded by white 

 matter, as in the medulla, or form a continuous peripheral layer, 

 as in the cerebrum (Fig. 64). It is impossible to give here a 

 detailed account of the functions of the parts of the brain, but the 

 more important localizations may be given. 

 The medulla, into which the spinal cord merges, serves as a 



Fig. 64. — Diagrams of cross-sections of different regions of the central nervous 

 system to show the distribution of white and gray matter. The regions of 



gray matter are stippled. 



A, spinal cord; B, medulla; C, cerebrum. 



pathway between the cord and other parts of the brain. It also 

 contains the centers that control the reflexes of the tongue and of 

 breathing. In the case of the tongue reflexes, afferent neurones 

 enter over the fifth and ninth cranial nerves, and motor neurones 

 pass out along the twelfth nerve. The adjuster neurones lie 

 within the medulla. The respiratory reflex depends upon the 

 sensitivity of the respiratory center in the medulla to the amount 



