74 HUMAN PHYSIOLOGY. 



turbance in the nerve-cells, acts upon the nerve-fibers in connection with 

 them. In the case of sensor or afferent nerves the stimuli act upon the pecu- 

 liar end organs with which the sensor nerves are in connection, which in turn 

 excite the nerve-fibers. Experimentally, it can be demonstrated that nerves 

 can be excited by a sufficiently powerful stimulus applied in any part of their 

 extent. 



Nerves respond to stimulation according to their habitual function; thus, 

 stimulation of a sensor nerve, if sufficiently strong, results in the sensation of 

 pain; of the optic nerve, in the sensation of light; of a motor nerve, in con- 

 traction of the muscle to which it is distributed; of a secretor nerve, in the 

 activity of the related gland, etc. It is, therefore, evident that peculiarity of 

 nerve function depends neither upon any special construction or activity of 

 the nerve itself, nor upon the nature of the stimulus, but entirely upon the 

 peculiarities of its central and peripheral end organs. 



Nerve stimuli may be divided into 



1. General stimuli, comprising those agents which are capable of exciting a 

 nerve in any part of its course. 



2. Special stimuli, comprising those agents which act upon nerves only 

 through the intermediation of the end organs. 



General stimuli: 



1. Mechanical: as from a blow, pressure, tension, puncture, etc. 



2. Thermal: heating a nerve at first increases and then decreases its excita- 

 bility. 



3. Chemic: sensor nerves respond somewhat less promptly than motor nerves 

 to this form of irritation. 



4. Electric: either the constant or interrupted current. 



5. The normal physiologic stimulus: 



(a) Centrifugal or efferent, if proceeding from the center toward the 



periphery. 



(&) Centripetal or afferent, if in the reverse direction. 

 Special stimuli: 



1. Light or ethereal vibrations acting upon the end organs of the optic nerve 

 in the retina. 



2. Sound or atmospheric undulations acting upon the end organs of the 

 auditory nerve. 



3. Heat or vibrations of the air upon the end organs in the skin. 



4. Chemic agencies acting upon the end organs of the olfactory and gustatory 

 nerves. 



Nature of the Nerve Impulse. As to the nature of the nerve impulse 

 generated by any of the foregoing stimuli either general or special, but little 

 is known. It has been supposed to partake of the nature of a molecular 



