122 THE MECHANISM OF LIFE 



from which the motor fibres of the cranial nerves issue. Keflexes, 

 or purposeful and useful adaptive movements, may be carried 

 out under such control, and apart altogether from the activities 

 of the brain, although there are probably very few actions into 

 which some amount of brain control does not enter. So far we 

 have supposed that the stimulus to an action is some change in 

 the environment acting on receptor organs situated in the skin, 

 but in the intact, normal animal it is far more likely that the 



Mfd Infra- cerebral 

 bram ^/pafh, cerebellum 



to rnid-u 

 Cerebellum 



-Afferent ftaih. 



Cord To cerebellum 



-Afferent f}a1h, 

 cord To lhalamus 



and m id- bra in 



---Efferent fait,, 

 mid- brain fd 

 Cord 



FIG. 35. AFFERENT IMPULSES FROM THE SKIN ARE SHOWN GOING UP 

 INTO THE THALAMUS, WHERE VISUAL IMPULSES ARE ALSO RECEIVED. 



Other afferent impulses enter the cerebellum from the muscles. From 

 the cereballum impulses pass to the mid-brain, and from there motor 

 impulses pass down through the cord, and to the muscles concerned 

 in locomotion. 



receptors stimulated will be one or several of the great cranial 

 organs of special sense. Thus an animal usually responds to 

 something which it sees, or hears, or smells, and so we must 

 include among the afferent paths that take part in the reflexes, 

 or other actions, those going from the visual, auditory, and 

 olfactory organs into the mid-brain. Yet the impulses actually 

 setting the muscles in action must issue from nuclei in the medulla 

 and spinal cord, and must go out to the muscles via the motor 



