408 



ORGAN SYSTEMS OF MAN 



more concern than the wearing of glasses 

 and it is gratifying to see these devices 

 gradually being accepted socially. 



Sense of balance 



The sense of body position, both static 

 and dynamic, is very important to an ani- 

 mal. The significance of this statement can 

 be observed by watching the movements of 

 an animal in which the balancing organs 

 have been destroyed or by observing a per- 

 son who has defective organs of equilib- 

 rium. In either case, when the eyes are 

 closed there is loss of control in maintaining 

 normal body position. What, then, are the 

 organs that control balance and body posi- 

 tion and movement? 



There are three vastly different organs 

 which have to do with tlie sense of balance 

 and movement. The eyes, when functioning 

 as indicated in the preceding paragraph, 

 operate continually in this capacity. A sec- 

 ond very important organ of equilibrium 

 is a portion of the inner ear, the non-acous- 

 tic labyrinth, while the third organ is the 

 system of proprioceptors, which are tiny 

 sensory endings located in the muscles and 

 tendons. Let us consider the non-acoustic 

 labyrinth and proprioceptors a little further. 

 The non-acoustic labyrinth is composed 

 of three semicircular canals and two small 

 chambers, the utriculus and the sacculus 

 (Fig. 16-11). While experimental evidence 

 now shows clearly that even fishes hear, the 

 cochlea is developed only in the higher ver- 

 tebrates. In all vertebrates, moreover, the 

 inner ear includes the organ of equilibrium. 

 The parts of tlie non-acoustic labyrinth 

 operate differently. The semicircular canals 

 function when change in rate or direction 

 of movement of the head occurs, and the 

 utriculus and sacculus have to do with the 

 position of the head. 



The three canals on each side are semi- 

 circular tubes arranged so that each is at 

 right angles to the others (Fig. 16-11); the 

 importance of this arrangement becomes 

 apparent when it is considered that no mat- 



ter how the head is moved two of the canals 

 (one in each ear) function. They are fluid- 

 filled and each has a small swelling called 

 the ampulla which houses a tuft of hair 

 cells that are sensitive to the movement of 

 the fluid. Any acceleration or deceleration 

 of head movement causes the fluid to flow 

 in definite directions in the canals, thus 

 stimulating the hair cells to send impulses 

 to the brain which results in the sensation 

 of movement. When this occurs in a hori- 

 zontal plane — the one people are accus- 

 tomed to — no particularly unpleasant sen- 

 sation results. However, if the head is 

 moved in a vertical plane, such as in the 

 abnormal movements of flying or riding in 

 an elevator, some very unpleasant sensa- 

 tions are experienced; in fact, they may 

 become so distasteful that nausea occurs, 

 as in seasickness. Just why such movements 

 should affect the stomach and bring about 

 disagreeable feelings is not very clear. For- 

 tunately, one can usually become accus- 

 tomed to such movements so that eventu- 

 ally there is no more response to them than 

 to horizontal movement. 



Head position, that is, static position, is 

 determined by the utriculus and sacculus. 

 These two chambers are also filled with a 

 fluid, and, in addition, each contains a tiny 

 lime pebble (the otolith) attached to the 

 sensory hair cells. Since the otolith is free 

 to move in the chamber, when the head 

 changes its position the pull of gravity shifts 

 the tiny weight so that the hairs are bent 

 in synchrony with its movement (Fig. 

 16-11). This is very similar to the operation 

 of the statocyst in the crayfish (p. 218). It 

 is the perception of this movement that 

 makes a person conscious of the position of 

 the body with respect to gravity. 



Not only is it important to register the 

 position and movement of the body as a 

 whole but, if coordination of all the com- 

 plex movements is to be had, there must be 

 some way of bringing about this interrela- 

 tionship. This is done by the hundreds of 

 proprioceptors, tiny sensory endings lo- 



