n6 TEXT-BOOK OF PHYSIOLOGY. 



into an active condition act for the most part on either the central or 

 peripheral end of the nerve. In the case of motor nerves the stimulus 

 to the excitation, originating in some molecular disturbance in the 

 nerve-cells, acts upon the nerve-fibers in connection with them. In 

 the case of sensory or afferent nerves the stimuli act upon the peculiar 

 end-organs with which the sensory 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 func- 

 tion; thus, stimulation of a sensory nerve, if sufficiently strong, re- 

 sults in the sensation 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 secretory nerve, in the activity of the related gland, 

 etc. It is, therefore, evident that peculiarity of nerve function de- 

 pends neither upon any special construction or activity of the nerve 

 itself nor upon the nature of the stimulus, but entirely upon the pecu- 

 liarities 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 endporgans. 



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 



composition 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 



gustatory nerves. 

 For efferent nerves 



A molecular disturbance in the central nerve-cells from which 

 they arise, the nature of which is unknown. 



