164 THE HUMAN BODY 



happening at a distance. The bodily adjustments based on them 

 are therefore long range adjustments rather than immediate ones. 

 Other adaptations than the simple one of withdrawing from a 

 source of injury are possible. Notably reflexes concerned with the 

 quest for food, a quest based in most animals largely on the senses 

 of smell and sight, are added to the purely protective reflexes. 

 Associated with these long range adjustments is a great group of 

 movements which constitute our most frequent and, in general, 

 most important muscular acts, movements of locomotion. These 

 make up a separate class of reflexes and will be studied by them- 

 selves next in order. 



The Sensory Basis of Locomotion. Locomotion takes various 

 forms in individual animals and in different classes of animals. 

 Walking, running, leaping, swimming, flying, these are all fun- 

 damental locomotor acts. With them must be classed also the 

 more artificial forms of locomotion of civilized man, as bicycle 

 riding or aviation. All these have certain primary features in 

 common. They all require the accurately co-ordinated use of a 

 number of muscles, and all of them involve the maintenance of 

 equilibrium. More, in fact, than the simple maintenance of balance 

 is involved. In every sustained locomotion there is constant 

 restoration of an equilibrium that is continually disturbed. Of 

 great importance for the guidance of co-ordinated muscular move- 

 ment is a sense whose receptors are embedded within the muscles 

 themselves and distributed about the joints to which the contract- 

 ing muscles impart movement. This is the muscle and joint sense, 

 or briefly, muscle sense. Less well known than some of our other 

 senses it is, as we shall learn (Chap. XIII), of equal rank with the 

 others, and in connection with our muscular movements more 

 important than most. Every bodily movement results in stimula- 

 tion of the receptors of muscle sense. Any locomotor act is accom- 

 panied by a great stream of impulses from these receptors which 

 serve not only to guide but to maintain the activity. There are 

 definite organs of equilibrium, the semicircular canals and vestibule 

 of the ear (Chap. XIV), by which the equilibrium sense is mediated. 

 These two senses constitute the essential sensory basis for the 

 locomotor reflexes. They are reinforced and modified by some of 

 the other senses, notably touch and sight. Since the locomotor 

 reflexes require the co-operation of several senses they are more 



