THE SENSES 571 



localising interior pain is, that the segment of the spinal cord 

 supplying the affected organ refers the pain to the skin region 

 of the same spinal segment instead of to the organ. Painful 

 sensations are of various characters — hence such terms as 

 1 stabbing,' ' boring,' ' burning,' ' throbbing,' etc., to express the 

 impression imparted. It is presumed that amongst the lower 

 animals these different qualities of pain exist ; it is quite certain, 

 for instance, that the pain exhibited by a horse during an attack 

 of colic is very different from that shown when pus is forming in 

 the foot. Of the nature of pain nothing is known. Its cerebral 

 centre lies in the thalamus (see p. 491). 



Muscle, Motorial, or Kinaesthetic Sense. 



The term ' muscle - sense ' has been employed to describe 

 several allied, though quite distinct, conditions to which the 

 muscles directly or indirectly contribute. It covers such 

 questions as a knowledge of the existence of muscles and of 

 their position ; the position of the limbs at rest and in 

 motion ; the proper and orderly contraction of muscles, and 

 of the correct grouping of sets of muscles — viz., muscular 

 co-ordination — both for locomotion and for equilibrium. It 

 goes further : it deals with weight and the resistance offered in 

 muscular contractions, and in conjunction with touch and sight 

 it forms conceptions of space, distance, and height. To many 

 it has appeared that the grouping of this set of complex functions 

 under the term ' muscle-sense ' is liable to prove misleading, for 

 there is no evidence that in all cases the muscles play a pre- 

 dominant part in the various phenomena ; in consequence 

 the term motorial or kinaesthetic sense is considered a more 

 suitable designation. 



Sherrington has shown that muscles receive a rich supply of 

 afferent nerves, one-half to one-third of the nerves in muscle 

 being of this nature. These sensory fibres terminate in special 

 nerve-endings in the muscle or its tendon. These endings are 

 known as muscle-spindles; they are believed to be activated 

 by variations in the tension of the muscles. The proof that 

 these spindles are sensory in nature is afforded by the fact 

 that when the inferior roots of the spinal cord are divided no 

 degeneration of them occurs. It is through these fibres, which 

 enter the cord by the dorsal roots and travel both to the 

 cerebrum and cerebellum, that the brain is made acquainted 

 with the condition of the muscles. The cerebellum, through its 

 connection with the semicircular canals of the internal ear, 

 enables a judgment to be formed of the position of the body in 

 space, and its relationships to its surroundings. These are com- 



