SENSATIONS OF MOVEMENT AND POSITION 671 



THE SENSE OF RELATIVE POSITION, INCLUDING THE 



MUSCULAR SENSE 



Without using our eyes we are able at any moment to tell the 

 position of our limbs. If one arm be moved passively into any 

 position we can without difficulty move the other arm into an exactly 

 similar position. We thus know the extent to which we move the 

 limb and the static position attained as the result of the movement. 

 If the movement is resisted, we are able to adjust the force of the 

 muscular contraction to the resistance, and to form therefore a fail- 

 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 sensation. Cutaneous sensa- 

 tions 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. 



(6) 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 



