THE STRUCTURE OF THE NERVE SYSTEM 343 



transmit impulses from the peripheral surfaces and 

 organs of the body to the brain and spinal cord, which 

 create conscious sensations or stimulate other reflex 

 activity. The former are termed efferent or centrifugal 

 (mostly motor nerves), the latter afferent or centripetal 

 (mostly sensor nerves). 



Physiology of Nerves. Nerves possess the function 

 of developing and conducting nerve impulses from 

 the nerve centres in the brain and spinal cord to the 

 periphery of the body, and at the same time to trans- 

 mit nerve impulses from the periphery to the centres 

 in the brain and spinal cord. As long as a nerve 

 is capable of these qualities it is termed excitable 

 or irritable, or possessed with irritability or excita- 

 bility. 



XERVE STIMULI. Xerves must receive some form 

 of external stimulation before they will develop or 

 convey nerve impulses, as they do not possess the 

 property of spontaneously developing and sending 

 out nerve impulses. A stimulus to motor nerves 

 (efferent), which excite it to activity, arise as a result 

 of some molecular disturbance within the nerve cells, 

 that acts upon the nerve fibers in connection with 

 them. In the case of sensor nerves (afferent) the 

 stimulus arises in the end-organs, which convey the 

 nerve impulse to the sensor nerve fibers in connection 

 with them. 



Xerves react to stimulation according to their 

 habitual function and distribution. If we stick our 

 finger with a pin, the sensation of pain is felt, due to 

 the fact that a sensor nerve has conveyed the nerve 

 impulse, started in the end-organs in the skin, to the 

 conscious centres in the brain ; stimulation of the end- 

 organs in rods and cones in the retina of the eye give 

 rise to the sensation of light; stimulation of a motor 

 nerve is followed by the contraction of a muscle 

 which it innervates. Xerve function is supposed to 



