io6 VETERINARY PHYSIOLOGY 



hunger are examples of these, sensations which, although due 

 to changes in the mouth, throat, or stomach, give us information 

 as to the general needs of the body. Such sensations may be 

 considered as normal and physiological. But when abnormal 

 conditions exist in certain localities they produce sensations 

 such as tickling, tingling, etc., and generally lead to a reflex 

 endeavour to remove the abnormal stimulus. 



When nocuous stimuli act they generally not only evoke 

 some reflex adjustment, as when indigestible matter in the 

 stomach causes vomiting, but they produce changes in the 

 consciousness which are generally classed as unpleasant or 

 painful. All pain, since it means a change in consciousness, 

 is metaphysical. There is not such a thing as " physical pain." 

 The fatigue and other sequences to any kind of pain are 

 frequently cited as proofs of the influence of the mind on the 

 body. But we have no right to assume that they are caused by 

 the " pain " rather than by the physical disturbances in the 

 nervous system of which the pain is an accompaniment. 



It must be recognised that pain is purely a relative term, and 

 that conditions which in one individual will cause pain will not 

 cause it in another, while stimuli which will produce what are 

 called painful sensations when the nervous system is debilitated 

 may give rise to sensations not considered as painful when the 

 nervous system is normal. 



The mechanism of common sensibility is not acted on by the 

 same stimuli in all parts of the body. The mouth and throat 

 are the parts to which the sensations are referred in abstinence 

 from fluids, the stomach in the absence of food. The intestine 

 appears to give rise to sensation only when abnormally 

 stimulated. In the skin the mechanism of common sensibility 

 is so closely associated with the mechanism of the tactile and 

 temperature senses that it is difficult to differentiate them. 

 Abnormal stimulation of the skin produces painful sensations 

 very readily, while similar changes in other tissues e.g. 

 muscles cause no modification of consciousness. 



The nerve channels by which the changes producing common 

 sensibility are transmitted in the central nervous system are 

 distinct from those connected with the tactile and other 

 senses, and common sensibility may persist while the tactile 

 sense is lost (see p. 109). 



