EQUILIBRIUM 139 



The muscle sense is the means by which the motions are 

 thus adjusted. In ourselves little attention is paid to it 

 because the sense works so perfectly that we do not find 

 it necessary to attend to it. In case of injury to the 

 sense, as for example, in the disease known as locomotor 

 ataxia (Chap. XVI), the impairment of locomotion is 

 very striking. 



Equilibrium. — This is a sense found in all animals 

 that have a special position, to which they will return 

 if disturbed. For example, so lowly an animal as a 

 starfish, if turned on its back, will with considerable 

 labor right itself. Insects, Crustacea, and all vertebrate 

 animals, have this equilibrium sense, and in all higher 

 animals there is found a special equilibrium organ. In 

 man and in all vertebrates this organ is located in the ear, 

 the semi-circular canals making up the chief portion of it. 

 It is an organ which is affected by shifts in the position 

 of the body with relation to gravity, and the animal in 

 which the shifts occur makes righting motions until it is 

 back in its accustomed position. 



The Contact Senses. — These are the senses which 

 are aroused by environmental conditions at the surface 

 of the body. They include touch, temperature, and the 

 chemical sense, so-called, as well as pain. The contact 

 senses have an interesting relationship to the activities 

 of animals in that they make up the original basis of the 

 relations of animals to their environment. All animals 

 are subject to those influences which come into direct 

 contact with them. It is only by secondary development 

 that means for coming under the influence of conditions 

 at a distance come into play. Further reference to this 

 interesting fact will be made later. 



Touch. — The sense of touch has a vast number of 

 organs consisting of tiny structures embedded in the skin 

 and distributed over nearly all parts of it. These organs 

 are much more abundantly present in some parts than 

 in others, but about the only surface of any size from 



