xvi THE NERVOUS SYSTEM 317 



as in illustration of such a chain-reflex. The optic reflex 

 of the moving fly produced the snapping reflex ; the contact 

 of the mouth epithelium with the fly produces the swallow- 

 ing reflex. Each of these reflexes is purely segmental. 

 By taking into account the act of transmission, com- 

 plicated acts can thus be resolved into a few segmental 

 reflexes." 



The action of the lower nerve centers is much influenced 

 by the inhibitory action of the parts lying in front of them. 

 That a frog with the optic thalamus or optic lobes removed 

 loses much of its spontaneity is due not to the fact that these 

 parts contain the centers for locomotion, but to the inhibiting 

 influence of these organs on the lower nervous centers. 

 This is shown by the fact that if the brain is removed still 

 farther back so as to include the anterior part of the medulla, 

 spontaneous movements will again make their appearance. 

 One of the most important functions of the higher nerve 

 centers is that of checking the movements of the lower 

 centers that would otherwise occur. In this way the frog's 

 actions are subject to a control which makes them more 

 adequately subservient to the needs of the animal. Locomo- 

 tion may be effected by the nerve centers of the spinal cord, 

 but when locomotion is set up, and when stopped, and how 

 it is directed are determined by impulses from the higher 

 nerve centers in response to sights, sounds, etc., from the 

 organs of special sense. The higher centers of the brain are 

 comparable to the captain of a steamer who issues orders to 

 the man running the engine when to start and when to stop, 

 and who has his hand on the wheel so as to guide the course 

 of the vessel. The machinery of locomotion works away by 

 itself, but it works blindly. The captain sees and avoids the 

 obstacles in the way and determines the direction and the 

 limits of the journey. In a similar manner the reflex 



