648 PHYSIOLOGY 



passive movement the chief end organs involved are those in connection with 

 the joints and their ligaments, though it is probable that the deeper sense 

 organs in the soft parts around the joints also contribute to the total sensa- 

 tions. Cutaneous sensations apparently play but little part in the j udgments 

 of passive movement. It is true that the alternating movements of the hind 

 limbs, which occur in a spinal animal when it is held up by the hands under 

 the fore limbs, are started, partly at any rate, by the stretching of the skin 

 of the thighs ; but this effect is one rather of initiation of movement, and can 

 hardly be regarded as proprioceptive in character. 



The strength of the sensation of passive movement depends on the 

 extent of the movement as well as on the rate with which it is carried out. 

 The delicacy of perception varies in different joints. Thus in some joints 

 a movement of 0'25 per second is appreciated as a movement, while in other 

 joints the movement must be as extensive as 1*4 per second. It is more 

 easily appreciated when the joint surfaces are pressed together than when 

 they are pulled apart, showing that the nerve-endings in the joint surfaces 

 play a part in the origination of the sensations. 



(b) THE SENSE OF MOVEMENT (MUSCULAR SENSATION). This term 

 is applied to those sensations by which we judge of the extent and force of 

 any active movement which we may have carried out. Many authors have 

 ascribed an important part in this act of judgment to the so-called ' sense of 

 innervation,' i.e. a sense of the actual energy which is being discharged 

 from the motor cells of the central nervous system to the muscles, and have 

 thought that when we raise a weight we judge of its amount, not by the 

 degree of stretching of the muscle or pressure on sensory nerves in the muscle, 

 but by the amount of force we voluntarily put out to raise the weight. The 

 fact however that we can judge of weights, when the muscles are made to 

 contract by electrical stimuli and not by voluntary impulses, shows that this 

 sense is in large part, if net entirely, peripheral. It is however very com- 

 plex in nature, and is served by a whole array of different end-organs in 

 the skin, joints, tendons, and muscles. The muscles themselves are known 

 to be well supplied with afferent nerves. Stimulation of the central end 

 of a muscular nerve may reflexly excite or inhibit movements of other 

 muscles. Sherrington has shown that, after section of the motor roots. 

 over one- third of the fibres in a muscular nerve remain undegenerated. 

 proving their connection with the posterior root ganglia. The sensory nerve- 

 endings in the muscle are represented partly by the tendon nerve endings 

 and partly by tin* muscle spindles. The former are richly branched end 

 arborisations of nerve fibres on the surface of the tendon bundles. The 

 muscle spindles consist of one or more muscle fibres, often continuous with 

 normal fibres, enclosed in a sheath composed of several layers of fibrous 

 tissue with intervening lymph >paces. One or more nerve fibres pierce this 

 sheath and, after making many spiral turns round the muscle fibres, branch 

 freely and terminate in lit t le knobs on the surface of the fibres (Figs. :\*2 1 .-:>'2'2). 

 The cross striation of the muscle fibres within the spindle is but faintly 

 marked. It is evident that the continuity of these sense organs with the 



