S44 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



sibly limit each of the feelings now classed together as " common sensations" 

 to definite anatomical structures. 



When the punctiform distribution of various sensations in the skin is inves- 

 tigated, some points are found in which no other sensation than that of pain 

 can be excited, and it has been thought that such spots mark the place of 

 ending of nerves of common sensibility. 



Transferred or "Sympathetic" Pains; Allochiria. It has long been a 

 matter of clinical observation that disease seated in certain internal organs is 

 often accompanied by superficial pain and tenderness in widely removed parts 

 of the body ; for example, a decayed tooth frequently causes intense pain in 

 the ear ; disease of heart or of aorta may cause pain between the shoulders, 

 etc. The subject has received most accurate investigation from Head, 1 who 

 has shown that there is an intimate nervous connection between the internal 

 organs and definite areas of the skin, manifested by pain and tenderness 

 appearing in sharply-localized regions on the surface when definite organs 

 become disordered. He has also demonstrated that disorders of the thoracic 

 and abdominal viscera not only produce pain and tenderness on the surface of 

 the body, but also cause pain and tenderness over certain areas of the scalp. 

 Head is inclined to explain the topographical association of skin-tenderness 

 with visceral disorders by the assumption that the nerve-supplies of the parts 

 so related find their origin within the same segment of the spinal cord. The 

 sensory result of visceral irritation may be summarized in the following way : 

 41 When a painful stimulus is applied to a part of low sensibility in close cen- 

 tral connection with a part of much greater sensibility, the pain produced is 

 felt in the part of higher sensibility rather than in the part of lower sensibility 

 to which the stimulus was actually applied." 



That this transferred localization may characterize other sensations than 

 those of pain has been definitely observed by Obersteiner, 2 who found that in 

 patients suffering from certain central nervous lesions tactile irritation of a cer- 

 tain point on the skin was referred by them to some other part of the body, 

 usually the corresponding point on the other side. He designated this trans- 

 ference of sensation by the term allochiria, meaning a confusion of sides. 



4. Muscular Sensation. Closely allied to common sensation, if not a 

 part of it, is muscular sensation. If two weights are to be compared, we 

 naturally do not lay them on the skin to determine their pressure-difference, 

 but we lift and weigh them in the hands, and experience shows that a much 

 more accurate estimate may thus be made. 



We undoubtedly have a keen perception of the tension of a muscle, and 

 therefore of the amount of resistance against which it is contracting. This per- 

 ception may be the outcome of a direct consciousness of the amount of motor 

 energy sent out from the motor cells, or it may be due to the inflow of sensory 

 impulses which show the tension to which the muscles have been subjected. 

 The latter view has more to be said in its favor. The sensory nerves involved 

 in this process are probably distributed rather to the tendons in which the mus- 

 1 Brain, 1893-94. 2 Ibid., 1881. 



