470 ORGANIC SENSATIONS 



This difference in terminology alone is evidence that views differ greatly as 

 to the real cause of these sensations. According to some authors, like Ch. Bell 

 and E. H. Weber, they are produced by excitation of the sensory nerves to the 

 muscles; others, like Lotze and Schiff, conceive that they are evoked by the 

 different foldings of the skin incidental to different positions; according to 

 Bernhardt the sensory nerves of the skin, of the fascias and of the periosteum 

 as well as the nerve trunks running through the muscles occasion muscular sen- 

 sations; Lewinsky seeks their cause in the excitation of the nerves of the joints 

 and bones ; and many authors like Leyden, Meynert, Nothnagel and others assume 

 that several different kinds of afferent nerves have a share in their production. 



As for lifting an object with the hand, in a great majority of cases we 

 send an impulse to the muscles which is exactly suited to the purpose, being 

 neither too weak nor too strong. That is, if the object is a familiar one, we 

 can adjust the voluntary impulse very exactly to the work to be performed. 

 From this the conclusion has been drawn that the feeling of effort is the 

 all-important thing in the perception of active movements (J. Miiller, Wundt). 



As a matter of fact it is easy to show that we do associate immediately with 

 a voluntary impulse an idea of the movement as if it were already performed. 

 Persons who have suffered amputation of a leg assert very positively that when 

 they will to bend the lost part they experience a distinct feeling that muscles 

 are being contracted. 



But the central feeling of effort, however important it may be, is not the 

 only determining factor. The mere development of our ability to adapt the 

 necessary motor impulses to the lifting of different objects, involves the con- 

 stant participation of afferent impulses which keep us informed of the results 

 of the impulses sent out. It can be shown also without difficulty that the 

 result of a voluntary impulse is usually controlled by afferent impulses. Thus, 

 if we misjudge the weight of an object e. g., overestimate it we give too 

 strong an impulse, as a consequence of which the object is lifted considerably 

 higher than we intended it should be and we are immediately aware of the 

 fact even without the use of our eyes. Similarly we are aware of the fact, 

 if the impulse is too weak. Naturally if these afferent impulses participate 

 in bringing about active movements, they must be the more necessary for 

 making us aware of passive movements. 



Let us see what are the afferent nerves mediating motor sensations. Ana- 

 tomical proof of the presence of afferent muscular nerves has been furnished 

 by Reichert, Kolliker, Odenius and others. We know too from the perfectly 

 definite sensations of fatigue as well as from the pains of muscle cramps that 

 these nerves are unquestionably able to mediate conscious sensations. They 

 also give rise to reflexes, among which those producing vasodilatation and 

 those involving the skeletal muscles as answering organs are the most impor- 

 tant (Tengwall). It seems probable therefore that these nerves do play a 

 prominent part in the motor sensations. 



In the case of the eye muscles the afferent nerves are of great importance. 

 We shall see later (Chapter XXI) that we have a very delicate appreciation of 

 the slightest contraction of the eye muscles. This could only be true, if afferent 

 nerves from the muscles or their tendons were present. 



