208 THE HUMAN BODY 



We have at any moment a fairly accurate knowledge of the 

 position of various parts of our Bodies, even when we do not see 

 them; and we can also judge fairly accurately the extent of a 

 movement made with the eyes shut. The afferent nerve impulses 

 concerned in the development of such judgments may be various; 

 different parts of the skin are pressed or creased; different joints 

 are subjected to pressure; different tendons are put on the stretch 

 and different muscles are in different states of contraction, and it 

 is by no means easy to determine the part played in each case by 

 the sensory nerves of the different organs. Moreover, when we 

 push against an object, or lift it, we are able to form a judgment 

 as to the amount of effort exerted; but here again pressure on 

 skin and joints and tension of tendons come in. Although under 

 normal circumstances the skin sensations are undoubtedly of im- 

 portance, they are not necessary: persons with cutaneous paralysis 

 can, apart from sight, judge truly the position of a limb and the 

 extent of movement made by it; and in many movements change 

 in joint pressure must be very little if any. We have then to look 

 to muscles and tendons themselves for an important part of the 

 sensations, and in both muscles and tendons there are organs in 

 connection with nerve-fibers which are certainly sensory in nature : 

 moreover, muscle sensory nerves appear to be excited by mere 

 passive change of form in the muscle; with the eyes closed each of 

 us can tell how much another person has lifted one of our arms. 



The sensations by which we judge the extent of a muscular 

 movement enable us to determine very minute differences of con- 

 traction; the ocular determination of the distance of an object not 

 too far off to have its absolute distance determined with con- 

 siderable accuracy, depends almost entirely upon judgments based 

 upon very small changes in the degree of contraction of the internal 

 and external straight (recti) muscles, converging or diverging the 

 eyeballs. A singer, too, must be able to judge with great minute- 

 ness the degree of contraction of the small muscles of the larynx 

 necessary to produce a certain tension of the vocal cords. It may 

 be well to point out that we do not refer a muscular sensation to 

 any given muscle or muscles; it is merely associated with a certain 

 movement or position, arid a person who knows nothing about his 

 ocular muscles can judge distance through sensations derived from 

 them, quite as well as any anatomist. This fact is of course cor- 



