102 TEXT-BOOK OF PHYSIOLOGY 



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 sensa- 

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

 in contraction 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 pecul- 

 iarity 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. 



The end-organs are specialized highly irritable structures placed between 

 the nerve-fibers and the surface. They are especially adapted for the 

 reception of special stimuli and for the liberation of energy, which in turn 

 excites the nerve-fiber to activity. 



General stimuli: 



1. Mechanic: Sharp taps, sudden pressure, cutting, etc. 



2. Thermic: Sudden application of heated object. 



3. Chemic: Contact of various substances which alter their chemic composi- 



tion quickly, e.g., strong acids or alkalies, sol. sodium chlorid 15 per 

 cent., sugar, urea, etc. 



4. Electric : Either the constant or induced current. 

 Special stimuli: 



For afferent nerves 



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 acting upon the end-organs in the skin. 



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



tory nerves. 



For efferent nerves 



A molecular disturbance in the central nerve-cells from which they arise, 

 the nature of which is unknown. 



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 

 disturbance, a combination of physical and chemic processes attended by 

 the liberation of energy, which propagates itself from molecule to molecule. 

 The passage of the nerve impulse is accompanied by changes of electric 

 tension, the extent of which is an indication of the intensity of the molecular 

 disturbance. Judging from the deflections of the galvanometer needle it is 

 probable that when the nerve impulse makes its appearance at any given 

 point it is at first feeble, but soon reaches a maximum development, after 



