328 THE VERTEBRATE BODY 



Balance is a very important sense which depends primarily upon two 

 different sets of sense organs. Within the inner ear there are three 

 semicircular canals which are so placed that each is at right angles to 

 the other two. The canals are rilled with a fluid and within them there 

 are delicate nerve endings which are sensitive to movement of the fluid. 

 When the position of the head is altered or when there is a sudden ac- 

 celeration or deceleration, the fluid will move most rapidly in the canals 

 which lie in the same plane as the movement. This moving fluid stimu- 

 lates the nerve endings and we are conscious of the movement even 

 though our eyes may be closed. There is another method which en- 

 ables us to determine the position of the head when it is not moving and 

 there is no movement of the fluid in the semicircular canals. There are 

 small stones of calcium carbonate, the otoliths, which are in a fluid-filled 

 sac, the sacculus, of the inner ear. The sac is lined with nerve endings 

 which are sensitive to contact with these bodies. The stones move by 

 gravity so that they always stimulate the nerve endings which are down. 

 This is similar to the action of the sand grains in the statocyst of the 

 crayfish. Also, the sense of balance is aided by a system of sense or- 

 gans, known as proprioceptors, which are located in the muscles and 

 tendons. These help to bring about the complicated series of reactions 

 which is necessary for the proper muscle coordination which we need 

 to maintain our balance. In fact, all coordinated muscular response is 

 in part dependent upon this set of sense organs. 



There is a variety of sense organs of the skin and other epithelial 

 tissues which may be grouped under the tactile sense. Some of these 

 are sense organs of touch which are sensitive to contact. Extreme stimu- 

 lation of some of these results in pain. Also, there are sense organs 

 of pressure, heat, and cold. These sense organs are not evenly dis- 

 tributed on all areas of the body — some simple experiments will show 

 that they are more numerous on the lips and finger tips than on areas 

 such as the shoulder or thigh. The advantage of such distribution is 

 easy to understand — we don't rub our shoulders against an object to 

 learn what it feels like. In addition to these the frog has some sense 

 organs in the skin which are sensitive to variations in the chemical en- 

 vironment — this enables him to recognize harmful chemical compounds 

 which may contact the skin. Alan's skin is so thick over most of the 

 body that this sense is lacking, but it may be found on the mucous 

 membranes which are found in the mouth and nasal cavities. You 

 can test this by peeling an onion or by holding a bottle of ammonia near 

 your nose. 



