554 PHYSIOLOGY 



deep sensibility ; unite with those which run in the protopathic system, 

 so that a lesion of the cord which abolishes the sense of pain will 

 abolish all forms of pain, whether arising from the skin or from 

 the underlying tissues. In the same way all temperature sensations, 

 whether the fine ones of the epicritic system or the coarser ones of 

 the protopathic system, run together in the cord. If the heat sense 

 is affected by a lesion of the cord all forms and all degrees of the sensa- 

 tion are affected in like measure, and the same applies to the sensations 

 of cold. 



THE HISTOLOGICAL CHARACTER OF THE ELEMENTS 

 INVOLVED IN CUTANEOUS SENSATIONS 



A very large number of different forms of sensory nerve-endings 

 have been described in relation to the skin. Their exact allocation 

 among the different cutaneous senses presents considerable difficulties. 



As regards touch, two kinds of elements are probably involved. 

 In the first place, the most sensitive tactile apparatus are the follicles 

 of the short hairs. Around these follicles we find a sheaf of nerve fibres, 

 some of which end in the hair papilla and others form a ring near the 

 level of the openings of the sebaceous glands. The other tactile end 

 organ is Meissner's corpuscle. The distribution of these in the skin 

 is not, however, dissimilar to that of the power of discrimination, with 

 which they may be specially connected. Other end-organs which are 

 supposed to be stimulated by changes of pressure, and therefore to 

 be tactile, are the organs of Ruffini which occur in the papillae of the 

 palm and fingers, and, lying more deeply, the elastic tissue spindles 

 as well as the Golgi corpuscles and the Pacinian corpuscles in the sub- 

 cutaneous tissue. 



As regards pain, we know that in the cornea, which possesses only 

 the pain sense, the sensory nerve-endings are in the form of branches 

 of axis cylinders among the epithelial cells. Similar free nerve- 

 endings occur in the epidermis all over the body, and it is therefore 

 imagined that these have the special function of subserving the pain 

 sense. We have at present no evidence as to the histological character 

 of the organs by which the sensations of heat and cold are aroused. 



