96 VETERINARY PHYSIOLOGY 



the threshold of effective stimulation. When the mechanisms 

 involved in the special senses are studied (p. 100 et seq.) it 

 will be found that special receptors are developed, each 

 variety of which lowers the threshold of excitability of the 

 arc for one kind of stimulus and increases it for all others. 

 It is by this arrangement that different kinds of change 

 each produce their appropriate result. 



Classification of Receptors. It has been already seen that 

 different reflexes of the cord may be evoked by different kinds 

 of stimulation from without. Different stimuli each call forth a 

 distinctive response. Thus, an injurious stimulus to the foot 

 causes a flexion and withdrawal of the leg, while pressure 

 between the toes causes an extension. Various receptors are, 

 as it were, tuned to special kinds of stimulation. Those which 

 respond to stimuli from without may be called extero-eeptive. 



They may be roughly classified into those responding to 

 injurious stimuli, noci-ceptive, and those responding to non- 

 injurio-us stimuli. Those connected with the cord respond to 

 stimuli resulting from changes in close proximity to the body, 

 such as touch, the addition or withdrawal of heat, the 

 application of chemical substances. 



But spinal reflexes are not only evoked by stimulation from 

 the outside of the animal, but also from stimulation of the 

 inside, e.g. stimulation of the inside of the stomach. The 

 receptors situated in these viscera may be called intero- 

 eeptive. 



Yet another set of stimuli come into play in spinal reflex 

 action. The end organs of neurons in the muscles, tendons and 

 joints may be stimulated by the reflex response of the muscles, 

 and may in turn set up reflex action often sustained and tonic 

 in character and of great importance in determining the posture 

 assumed. These receptors, stimulated by the condition of the 

 animal's own tissues, may be termed proprio-eeptive (see fig. 42). 



Stimulation of the extero-eeptive receptors plays the chief 

 part in initiating movements, stimulation of the proprio-eeptive. 

 as a result of these, in guiding and co-ordinating the movements. 

 The proprio-eeptive reflex when induced may reinforce the extero- 

 eeptive reflex which started it, or it may evoke compensatory 

 movements, bringing the part back to its former position. 



