CUTANEOUS AND INTERNAL SENSATIONS 1095 



with resistance to movement. In none of these groups are we dealing 

 with purely muscular sensations; cutaneous tactile sensations and 

 pressure sensations elicited from other structures than muscles are 



also involved. 



Voluntary muscular movements are accompanied with a peculiar 

 sensation of effort, graduated according to the strength of the con- 

 traction, and affording data from which a judgment as to its amount 

 and direction may be formed. 



It has been shown, however, that when pressure sensations are elimi- 

 nated or reduced to a minimum by enclosing the arm, held horizontally, 

 in a rigid apparatus such as that shown in Fig. 469, the moments of 

 rotation of a weight* fastened at different distances from the shoulder 

 can be discriminated v.'ith great exactness. The sensation of effort is 

 therefore an independent sensation (v. Frey). 



Fig. 469- Apparatus for Arm for Testing Muscle Sense. At the right is shown a lead 

 weight which can be placed on either of the hoops of the apparatus (v. Frey). 



Some writers have supposed that this so-called muscular sense does 

 not depend upon afferent impulses at all, but that the nervous centres 

 from which the voluntary impulses depart take cognizance, retain a 

 record, so to speak, of the quantity of outgoing nervous force ; that the 

 effort which we feel in lifting a heavy weight is an effort of the cells 

 of the motor centres from which the groups of muscles are innervated, 

 and not of the muscles themselves. 



But although this feeling of central effort or outflow (we can hardly say 

 of central fatigue) may be a factor, it cannot be doubted that the brain 

 is kept in touch with the contracting muscle by impulses of various 

 kinds which reach it by different afferent channels. 



The corpuscles of Pacini, which exist in considerable numbers in the 

 neighbourhood of joints and ligaments, and in the periosteum of bones, 



Fi?. 470. Nerve-Ending in Tendon near the Insertion of the Muscular 

 Fibres (Go}e\\. 



Fibres (Golgi). 



would seem well fitted to play the part of end-organs for the tactile 

 sensations caused by the movements of flexion, extension, or rotation 

 of one bone on another, which form so large a portion of all voluntary 



* With the arm horizontal the moment is the product of the weight by its 

 distance from the shoulder joint (see p. 749). 



