CUTANEOUS AND INTERNAL SENSATIONS 1053 



sensation of effort, graduated according to the strength of the con- 

 traction, and affording data from which a judgment as to its amount 

 and direction may be formed. 



Some writers have supposed that this so-called muscular sense does 

 not depend upon afferent impulses at all, but that the nervous centres 

 from which the voluntary impulses depart take cognizance, retain a 

 record, so to speak, of the quantity of outgoing nervous force; that the 

 effort which we feel in lifting a heavy 

 weight is an effort of the cells of the 

 motor centres from which the groups of 

 muscles are innervated, and not of the 

 muscles themselves. 



But although this feeling of central 

 effort or outflow (we can hardly say of 

 central fatigue) may be a factor, it cannot 

 be doubted that the brain is kept in touch 

 with the contracting muscle by impulses 

 of various kinds which reach it by different 

 afferent channels. 



The corpuscles of Pacini, which exist 

 in considerable numbers in the neigh- 

 bourhood of joints and ligaments, and in the periosteum of bones, 

 would seem well fitted to play the part of end-organs for the tactile 

 sensations caused by the movements of flexion, extension, or rotation 

 of one bone on another, which form so large a portion of all voluntary 

 muscular movements. And it has been stated that paralysis of these 

 bodies in the limbs of a cat by section of the nerves going to them 

 causes a characteristic uncertainty of movement which suggests that 

 something necessary to normal co-ordination has been taken away. 

 Tendons also possess afferent nerve-fibres, which terminate by breaking 



Fig. 447. Nerve - Ending in 

 Tendon near the Insertion of 

 the Muscular Fibres (Golgi). 



pr.c 



m.x.b 



Fig. 448. Muscle Spindle (after Ruffini). c, sheath of the spindle ; n.tr., trunk 

 of nerve, which sends fibres through the sheath into the spindle, where they 

 form endings (pr.e., s.e., pl.e.) of various kinds; m.n.b., bundle of motor fibres. 



up into reticulated end-plates (Fig. 447). We have already seen that 

 the skeletal muscles possess numerous afferent fibres (p. 910). Some 

 of these must be nerves of ordinary sensation. For, although when a 

 muscle is laid bare in man and stimulated electrically, the sensation 

 does not in general amount to actual pain, it is capable, under the 

 influence of strong stimuli, of taking on a painful character. And 

 nobody who has felt the severe and sometimes almost intolerable pain 

 of muscular cramp would be likely to deny the existence of sensory 

 muscular nerves. But after deducting these, we must assume that a 

 large proportion of the afferent nerves of muscle have other functions, 

 and among them may be the conveyance of impulses connected with 



