62 PHYSIOLOGY CHAP. 



were special organs destined exclusively for sensations of pain, we 

 should be " driven to conclude that such ... a mechanism of pain 

 has been preserved intact but unused through whole generations in 

 order that it may once in a while come into use, which is in the 

 highest degree improbable. This difficulty disappears if we suppose 

 that the constantly smouldering embers of common sensibility 

 may be at any moment fanned into the flame of pain." 



So that, if we assume with v. Frey that there are numerous 

 pain spots in the skin with corresponding nerves and end-organs, 

 this does not mean that there is a specific apparatus exclusively 

 intended to serve pain sensibility ; it is the same that subserves 

 the common sensibility of the internal organs and tissues, and 

 normally transmits subconscious excitations only. Granting this 

 to be reasonable, it does not therefore exempt us from examining 

 whether the afferent nerves of the internal organs have normally, 

 like those of the external tegument, the capacity for arousing 

 pain, when artificially stimulated with excessively strong stimuli. 

 This question is very important from a practical point of view. 



The physiologists of the seventeenth and eighteenth centuries 

 were much occupied in testing the sensibility of the internal parts 

 to pain in animals, and many important surgical observations were 

 also made on man previous to the introduction of ether and 

 chloroform narcosis. The introduction in recent times of the 

 method of local anaesthesia, specially by cocaine for major 

 operations, has opened a new era in the study of this subject, 

 making it possible to test the sensibility of the different tissues. 

 But the results are at present contradictory or uncertain. 



The general results obtained from the whole of these observa- 

 tions, new and old, may be summed up as follows : 



(a) Only the tissues provided with nerves are sensitive to 

 pain stimuli: the epidermis, the horny tissues in general, the 

 cartilages and fibro-cartilages are totally insensitive, because they 

 have no nerves. 



(b) The organs, tissues, and internal membranes innervated 

 by the sensory roots of the nerves of the cerebro-spinal axis are 

 more or less sensitive to painful stimulation. 



(c) The organs and internal tissues innervated exclusively by 

 the nerve-fibres of the sympathetic system are little sensitive to 

 pain stimuli under normal anatomical and functional conditions, 

 but in a state of inflammation they may acquire an exquisite 

 sensibility to pain. 



There are no exceptions nor comments for the first proposition ; 

 the second and third, on the contrary, must be examined. The 

 connective tissues, ligaments, tendons, and aponeuroses have, 

 under normal conditions, an indefinite sensibility to pain. The 

 periosteum is very painful, as shown on scraping the bones in 

 certain surgical operations ; but bone itself, particularly the 



