THE ORGANS OF SENSE. IO? 



degree of sensation bears a constant proportion to the strength of the 

 original stimulus. If the eye is being stimulated by the light of one 

 hundred candles, the extra stimulation required to produce the sensation 

 of more light can be derived from one more candle. If, however, the 

 original stimulus came from one thousand candles, then ten more candles 

 would be required for a difference to be detected. 



Similarly, if a weight of 30 grams is held in the hand, one more 

 gram must be added to excite a sensation of increased weight, and a 

 smaller increase would not be noticed. If the original weight is 60 

 grams, 2 grams must be added for the difference to be perceptible. So 

 also in the case of sound, the stimulus must be increased by one-seventh 

 to lead to the perception of increased sound volume. In all cases this 

 law, which is known as Weber's law, only holds good within certain 

 definite limits, the limits for the pressure sense as tested by weights 

 lying between 50 and 1000 grams. 



CUTANEOUS SENSATIONS. 



The sensations aroused by the application of different varieties of 

 stimuli to the skin are those of pressure, including tactile localisation 

 and tactile discrimination, heat, cold, and pain. These various sensations 

 are independent and do not result from different forms of stimulation 

 applied to the same set of nerve endings. The evidence that tactile 

 localisation, tactile discrimination, heat, cold, and pain are distinct senses 

 is (1) histological, (2) the existence of independent spots in the skin, 

 stimulation of each of which gives rise to one variety of sensation only, 

 and (3) the fact that interference with the conducting paths may 

 result in blocking of one set of impulses, for example, those giving rise 

 to pain, while the other senses are unaffected. 



Histological. The End-organs in the Skin. The peripheral fibre 

 from a cell in the posterior root ganglion or the homologous ganglion 

 on a cerebral nerve terminates in the skin in various ways. The 

 termination may be free, or it may be protected. Free nerve endings 

 are found in the anterior epithelium of the cornea ; the axis cylinder of 

 the nerve fibre loses its myelin sheath at the periphery of the cornea, 

 and, after forming a plexus in the corneal substance, the fibre terminates 

 between the epithelial cells in the form of fine varicose fibrils. Similar 

 terminations are found in the epidermis, and fibrillar nerve endings 

 occur also around the hair follicles. The protected nerve endings are all 

 formed on the same general plan. There is a central soft core 

 surrounded by a variable amount of fibrous tissue, arranged sometimes 

 irregularly, and sometimes, as in the Pacinian corpuscles, in laminae. 

 The nerve fibre loses its myelin sheath and runs into the core of the 



