88 HUMAN PHYSIOLOGY. 



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 efferent if proceeding from the centre toward the 

 periphery. 



(b) Centripetal or afferent if in the reverse direction. 



(c) Reflex, a combination of the two preceding. 



The Axis Cylinder is the essential conducting agent, the white substance 

 of Schwann and tubular membrane being probably accessory structures, 

 protecting the axis from injury, and preventing the diffusion of nerve force 

 to adjoining nerves. 



The properties of sensation and motion reside in different nerve fibres. 

 Motor nerves can be destroyed or paralyzed by the introduction of woorara 

 under the skin, without affecting sensation; the sensibility of nerves can be 

 abolished by the employment of anaesthetics without destroying motion. 



Irritability. Nerves conduct peripheral impressions to the centres, and 

 motor impulses to the periphery, in virtue of their possessing an ultimate 

 and inherent property, denominated neurility, nervous irritability ', or 

 excitability, which is manifested as long as the physical and chemical 

 integrity of the nerve is maintained. 



Nerve Degeneration. When nerves are separated from their trophic or 

 nutritive centres, they degenerate progressively in the direction in which 

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

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



Nerve force is not identical with electricity. Nerves do not possess the 

 power of generating force, or of originating impulses within themselves, 

 but propagate only the nervous impulses which are called forth by chemical, 

 physical and mechanical stimuli from without, and by volitional acts, normal 

 and pathological conditions from within. 



Velocity of Nerve Force. This is slightly modified by temperature, cold 

 lessening the rapidity, heat increasing it ; it is also modified by electrical 

 conditions, by the action of drugs and by the strength of the stimulus. The 

 average velocity for human sensory nerves has been estimated at about 190 

 feet per second, and for motor nerves 100 to 120 feet per second. 



Phenomena of Muscles and Nerves. The muscles are the motor 

 organs of the body and constitute a large per cent, of the body weight. 

 Muscles are of two kinds, striated and non-striated or involuntary. The 

 striated muscles consist of bundles of fibres, the fasciculi, held together by 

 connective tissue. Each muscle fibre is about ]/% to \y z inches long, and 



