984 A MANUAL OF PHYSIOLOGY 



of the afferent nerves of muscle have other functions, and among 

 them may be the conveyance of impulses connected with the mus- 

 cular sense. The muscle-spindles or neuro-muscular spindles 

 (Fig. 433), peculiar structures which occur in large number in most 

 of the skeletal muscles, and have been carefully studied by Huber, 

 Sihler, Ruffini, and other observers, are the terminations of many 

 of the sensory fibres. They are long narrow bodies, with a thick 

 sheath of connective tissue enclosing fine striped muscular fibres. 

 Medullated nerve-fibres enter the spindle, and there, dividing into 

 branches and losing their medullary sheath, form endings of various 

 kinds around and between the muscular fibres. It is possible that 

 in contraction of the muscles the nerve-fibres in the spindles arc 

 compressed, and thus mechanically stimulated. 



In the spinal cord these impulses are conducted up through the 

 posterior column ; and, although less is known as to the paths 

 they follow in the higher parts of the central nervous system, it is 

 certain that there is some afferent bond of connection between the 

 cortical motor areas and the muscles which they control (p. 856). 



Tactile sensations set up in the skin or mucous membrane lying 

 over contracting muscles may also help the nervous motor mechanism 

 in appreciating and regulating the amount of contraction ; but the 

 fact that, in anaesthesia of the mucous membrane covering the vocal 

 cords produced by cocaine, the voice is not at all impaired, shows 

 that muscular contractions of extreme nicety can be carried on 

 without any such aid. 



Relation of Stimulus to Sensation. 



It is impossible to measure sensation in terms of stimulus. All 

 that we can do is to compare differences in the intensity of stimuli 

 and differences in the resultant sensations, or, in other words, to 

 compare stimuli together and to compare sensations together. And 

 when we determine the amount by which a given stimulus must be 

 increased or diminished in order that there may be a just perceptible 

 increase or diminution in the sensation, it is found that (with certain 

 limitations) the two are connected by a simple law : Whatever the 

 absolute strength of a stimulus of given kind may be, it must be in- 

 creased by the same fraction of its amount in order that a difference 

 in the sensation may be perceived (sometimes called Weber's law}. 

 Thus, a light of the strength of one standard candle must be in- 

 creased by T ^oth candle, a light of 10 candles by y^yths, and a light 

 of 100 candles by a candle, in order that the eye may perceive that 

 an increase has taken place, just as the weight necessary to turn a 

 balance increases with the amount already in the pans. The frac- 

 tion varies for the different senses. It is about y^y for light, for 

 sound. But it would appear that Weber's law does not hold for 

 the pressure sense, nor for the other senses above and below certain 

 limits. Fechner, making various assumptions, has thrown Weber's 



law into the form y=k - , where y is the intensity of sensation, 



XQ 



x the intensity of stimulation, X Q the smallest intensity of stimulus 

 which can be perceived (liminal intensity), and k, a constant. This 

 so-called psycho-physical law of Fechner states that the sensation 

 varies as the logarithm of the stimulus. But Fechner 's law has 

 been subjected to serious criticism, and the subject cannot be 

 further pursued here. 



