MONITORS 219 



nerve-endings are essential in making co-ordinated move- 

 ments of the different muscles as, for example, in grasping 

 an object between the fingers and thumb, or in moving the 

 legs when walking. Thus a horse can be taught to walk in 

 a gymkhana along a row of bottles set out on the ground, 

 stepping in the spaces between them and not knocking one 

 over, although he cannot see his hind feet — he has seen the 

 bottles and his muscle sense tells him where his feet are in 

 relation to them. Even on a cold winter's day when the feet 

 are numb through walking through the snow we know where 

 they are and one can go on putting one before the other — 

 the sense of touch has been put out of action by the cold, but 

 the muscle sense in which the nerve-endings are not so near 

 the surface is unaffected. 



The proprioceptive sense that is served by part of the 

 internal ear tells us which way up we are and how we are 

 changing direction when we move — part of it is a gravity- 

 receptor and part of it an angular-acceleration-receptor. In 

 all the vertebrates, except a few of the most primitive such 

 as the lamprey, the structure of these receptors is essentially 

 similar. It consists of two small membranous bags connected 

 with three semicircular canals arranged in planes at right 

 angles to each other. They are all embedded in the bone of 



AMPULUE 



SEMICIRCULAR 

 aNALS 



r SUPERIOR 

 LATERAL 

 I POSTERIOR 



COCHLEA 



Fig. 18. A diagram of the three semicircular canals arranged at right angles to 

 each other. The ampullae of the canals open into one of the membranous 

 bags (the utriculus) which is connected via the second bag (the secculus) to 

 the cochlea. The sausage-shaped bag below is the endolymphatic sac. 



