MONITORS 221 



each other; the three thus He in the three planes that are 

 seen, for example, where three sides of a box meet at any of 

 the corners. As a result of this arrangement movement of the 

 head in any direction sets the fluid moving in one or more 

 of the canals, and the sensory cells are stimulated. At first 

 sight one might imagine that two canals, one vertical and 

 one horizontal, would be enough to give an indication of 

 head movements in any plane. If, however, there were only 

 two, a movement about the axis of the junction of the two 

 planes would not produce any movement in the fluid, 

 whereas with three there is no possible movement that would 

 not cause the fluid to flow. There are, in fact, some animals, 

 the lampreys, that have only two canals, not, as one might 

 expect, vertical and horizontal, but both vertical — it is the 

 horizontal one that is missing. Yet lampreys apparently 

 swim as efficiently as the fishes, move up or down in the 

 water, turn from side to side and keep on an even keel, as 

 well as animals with three canals. 



It may be that the lampreys use their eyes to help them 

 in keeping their balance. If the canals are put out of action 

 in a pigeon, for example, the bird cannot stand, walk or fly, 

 but is in constant unco-ordinated movement. After a time, 

 however, it regains partial control over its movements and 

 appears more or less normal. If then its eyes are covered it 

 is as bad as ever; the eyes have evidently taken over some 

 of the functions of the canals, and it relies on visual images 

 to orientate itself. Similarly the lamprey may use its eyes to 

 provide the information that it lacks through the absence of 

 the horizontal canal. To put against this suggestion there is, 

 however, the case of the lamprey's near relation, the Hag 

 fish. This creature has only one semicircular canal and no 

 eyes, and yet it appears to suflfer no handicap. Nobody knows 

 the answer to this riddle. 



In the invertebrate animals we have seen that the ear 

 differs widely from that in the vertebrates. The proprio- 

 ceptive balancing sense although basically the same also 

 differs greatly in detail. In the first place there is usually no 

 separation of the part used as a gravity-receptor from that 

 for appreciating angular acceleration. The conspicuous 

 exception to this generalization is found in the octopus, 



