THE REGULATING MECHANISM 185 



sensory nerve may either be the sensory nerve 

 of the skin and of the eyeball (the fifth cranial 

 nerve), or the optic nerve itself through the 

 retina, while the motor nerve is a branch of 

 the seventh cranial nerve, the facial, supplying 

 the muscle that closes the eyelids (the orbicu- 

 laris palpebrarum). The movements of swal- 

 lowing and the respiratory movements are 

 examples of highly complex reflex actions, 

 involving many nerves and many muscles. 

 We may or may not be conscious of reflex 

 movements, but they cannot be arrested by 

 an effort of the will. Many movements, at 

 first consciously performed, become reflex 

 without consciousness, as in locomotion, play- 

 ing on an instrument and working a machine. 

 The centres are found in the brain and cord. 

 103. The nervous mechanisms we have con- 

 sidered cause increased activity. It is prob- 

 able that even while apparently at rest 

 molecular phenomena are occurring in nerve 

 cells. Thus certain nerve fibres issuing from 

 neurones in the spinal cord pass to the muscles- 

 of the limbs and keep these in a state of partial 

 contraction or tonics, as it is termed. In this 



