PROPERTIES AND FUNCTIONS OF NERVES. 97 



or nutritive centres, they degenerate progressively in the direction in 

 which they conduct impressions. In motor nerves, from the centre to the 

 periphery; in sensory nerves, from the periphery to the centres. 



Stimuli of Nerves. Nerves do not possess the power of spontaneously 

 generating and propagating nerve impulses ; they can only be aroused to 

 activity by the action of an extra-neural stimulus. In the living condition, 

 the stimuli capable of throwing the nerve 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 fibres in con- 

 nection 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 connec- 

 tion, which in turn excite the nerve fibres. 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 sensory nerve, if sufficiently strong, results 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 pecu- 

 liarity of nervous 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 : 1st, General stimuli, comprising 

 those agents which are capable of exciting a nerve in any part of its course ; 

 2d, 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 



excitability. 



3. Chemical : Sensory nerves respond somewhat less promptly than motor 



nerves to this form of irritation. 



4. Electrical ; Either the constant or interrupted current. 



5. The normal physiological stimulus: 



(a) Centrifugal or afferent if proceeding from the centre toward the 



periphery. 

 (l>) Centripetal or afferent if in the reverse direction. 



