THE SENSES 



983 



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 inner- 

 vated, and not of the muscles them- 

 selves. 



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 neighbourhood 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 move- 



FIG. 432. NERVE-ENDING IN 

 TENDON NEAR THE INSER- 

 TION OF THE MUSCULAR 

 FIBRES (GoLGi). 



m.n.h. 



FIG. 433. MUSCLE SPINDLE (HALLIBURTON, 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. 



ment which suggests that something necessary to normal co-ordina- 

 tion has been taken away. Tendons also possess afferent nerve- 

 fibres, which terminate by breaking up into reticulated end-plates 

 (Fig. 432). We have already seen that the skeletal muscles possess 

 numerous afferent fibres (p. 835). 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 



