96 HUMAN PHYSIOLOGY. 



give rise to sensations ; reflective or excitant, when the impression carried 

 to the nerve centre is reflected outward by an efferent nerve and produces 

 motion or some other effect in the part to which the nerve is distributed. 



(2) Efferent or centrifugal, as when the impulses generated in the centres 

 are transmitted outward to the muscles and various organs. They may be 

 motor, as when they convey impulses to the voluntary and involuntary 

 muscles; vasomotor, when they regulate the calibre of the small blood 

 vessels, increasing or diminishing the amoimt of blood to a part ; secretory, 

 when they influence secretion ; trophic, when they influence nutrition ; 

 inhibitory, when they conduct impulses which produce a restraining or 

 inhibiting action. 



Irritability, excitability, neurility. All nerves possess the property of 

 being called into action by a stimulus in virtue of the possession of an 

 ultimate and inherent property denominated irritability or excitability, 

 which is manifested so long as the physical and chemical integrity of the 

 nerve is maintained. During the period of excitement, no change in the 

 nerve is appreciable except an electrical one. 



The irritability of a motor nerve is demonstrated by the contraction of 

 the muscles to which it is distributed; the impulse aroused by an irritant 

 travels outward to the muscles and calls forth a contraction. 



The irritability of a sensory nerve is demonstrated by the development 

 of a conscious sensation. An irritation applied to a sensory nerve in any 

 part of its course arouses an impulse which travels to the brain and produces 

 there a sensation. The irritability of a sensory nerve may be increased by 

 congestion or inflammation and decreased by cold, compression and injuries. 

 Other tissues e.g., muscles, glands, etc. possess irritability, and when 

 subjected to the action of a stimulus react in their own particular way. The 

 irritability of nerves is distinct and independent of the irritability of muscles 

 and glands, as can be demonstrated by the use of poisons, such as woorara, 

 atropia, etc. 



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 destroy- 

 ing motion. 



In the transmission of the nerve impulse 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. 



Nerve Degeneration. When nerves are separated from their trophic 



