154 ESSENTIALS OF PHYSIOLOGY. 



constitutes muscle sense, and is brought about by afferent impulses 

 constantly passing from special structures in the muscles, joints, and 

 ligaments. About one-third of the fibres in a nerve to a muscle are 

 afferent in function, and do not degenerate on section of the anterior 

 roots. These afferent fibres terminate in the muscles in neuro- 

 muscular spindles, lying between the ordinary muscle fibres. The 

 muscle spindles consist of fine muscular fibres surrounded by a con- 

 nective-tissue sheath. A nerve fibre loses its myelin sheath, pierces 

 .the covering of the spindle, and divides into bundles of fibrils which 

 make several spiral turns round the muscle fibres, and then end by ar- 

 borisation. Other nerve endings are found in tendon, and consist in 

 the arborisation of a nerve fibre around a bundle of tendon fibres. 



Sensation of passive movements is due chiefly to impulses arising in 

 the joints, that of active movements to impulses arising in the muscles. 

 It is through this sense that we are able to form an estimate of weight. 



The impulses giving rise to muscle sense also take part in the 

 co-ordination of muscular movement, as has already been described ; 

 and their absence may lead to disturbances of equilibrium. 



The maintenance of equilibrium is dependent, therefore, on afferent 

 impulses from the muscles and the labyrinth as well as from the eyes 

 and the skin ; when the sensations resulting from the impulses from 

 these different sources are discordant, we experience a feeling of 

 giddiness, and at the same time equilibrium is disturbed. 



