600 PHYSIOLOGY 



adjust the force of the muscular contraction to the resistance, and to form 

 therefore a fair idea as to the strength of the resistance. 



(a) PASSIVE MOVEMENTS. A large number of different sense-organs 

 contribute to the formation of these judgments. In the appreciation of 

 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- 

 tion. Cutaneous sensations apparently play but little part in the judgments 

 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 not 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 the 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-spaces. One or more nerve fibres pierce this 



