ioio THE MUSCULAR SENSE. 



muscle fibre when it contracts. It is true that change of form of the spindle 

 must occur to some extent, not only when the muscle shortens, owing to 

 contraction, but also when its points of attachment are approximated by other 

 causes {e.g. passive movement). But the change is, for the same degree 

 of approximation of its attachments, not quite the same when passively 

 obtained, either as to extent or character. The rectilinear course of the fibres 

 is not so maintained. When the ankle of the frog is fully plantar-flexed, the 

 sural muscle lies actually in folds. But the muscle spindle is peculiar among 

 sensory end-organs, in that the part to which the actual nerve-ending is applied 

 is not only contractile but extensile. Each lengthening and shortening of the 

 muscle, within degrees which do not overstep the limits of extensility and 

 elasticity of the muscle fibres, changes the figure of the intrafusal muscle 

 fibres. Rufnni's annulo-spiral ending is a form of nerve-ending special to 

 the spindle. There is no ground for thinking the nerve fibril as extensile and 

 elastic as a muscle fibre. The tendril-like rings of the nerve fibril must, if so, 

 be subject to strain when the muscle fibre they clasp becomes thicker, and 

 when they are pulled apart from each other as the muscle fibre elongates. It 

 is noteworthy, however, that in some cases the muscle fibre, to judge from its 

 less distinct striation there, is less contractile at the place of the nerve-ending 

 than elsewhere. The striated and fibrillated substance of the fibre is there 

 largely replaced by the voluminous nuclei I have described ; these lie in a 

 central core of finely granular material, surrounded only by a striated zone, 

 recalling the appearance of a Purkinje cardiac fibre. In some cases the 

 striation of the muscle fibres seems to fail at the nerve-ending, though in 

 many it is perfectly continuous throughout. 



Beside changes in form, changes in tension must also occur in the 

 spindles ; in the tendon organs the changes in tension must be much greater 

 than the change in form, the tendon bundle being but little extensible or 

 elastic. Its tension will wax and wane with the tension throughout the 

 muscle. The stimulus for the tendon organ may well be mechanical ; the 

 nerve-endings that penetrate the little tendon bundle (Golgi's tendinetta) of 

 the organ may be compressed as the muscle contracts or is passively stretched. 

 It is easy to understand how the organs might react under both passive and 

 active movements of the limb. The Pacini corpuscle is often interpreted as 

 being adapted for stimulation by compression. The modified form is more 

 usual in muscles than is the typical form. It occurs in some places, 

 e.g. sheaths of muscles, where there is likelihood of its being laterally 

 compressed. The ordinary Pacini, embedded in muscle, is admirably placed 

 for being compressed, especially when, as sometimes, seated in the retiring 

 angle between a septum or aponeurosis and obliquely inserted muscle bundles. 



From the above we might expect tension to largely determine muscular 

 sense perceptions. It must be admitted, however, that observations showing 

 that the liminal excursion for passive movements is practically unaffected by 

 the initial position of the joint, is against tension being of such great 

 importance. The same position or movement of a joint may involve very 

 different degrees of tension, according as it is favoured or hindered by outward 

 forces (traction of weight, etc.), including muscular ; yet the limen of judg- 

 ment remains the same ; the weighting of one of the compared hands makes, 

 according to Loeb, very little difference to the illusions he studied as to 

 excursion and direction of " willed " movements. This difficulty in the way 

 of regarding the perceptions of muscular sense as based on elemental sensa- 

 tions of tension, may be partly met by supposing that, while some of the sense 

 organs have tension for adequate stimulus, some have change of form with or 

 apart from tension. Impressions from tension-organs (isometric), checked against 

 simultaneous impressions from form-organs (isotonic) or organs of joints, might 

 supply a judgment based on their ratio, which might give the required ideas. 

 One and the same sensation of muscle-form or of joint-movement, summed with 



