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492 Bonnier. To-day it is known that their function is directly connected with the faculty of equilibration and is entirely independent of audition. M. P. Bonnier, after having studied throughout the entire animal series the functions of the labyrinth and those organs which precede it, by comparing the data of comparative anatomy and physiology and verifying them by clinical observations, has been able to show that these organs subserve directly what he calls the "sense of altitudes," which furnishes the images of position, of distribution, and consequently of movement and of displacement in space.

It is not yet exactly known what is the physiological excitant which puts in action the semicircular canals; awaiting further researches for the settlement of this interesting point, we will try to determine the method of action of the sense of direction. This way of procedure is moreover in no way illogical—in natural sciences, as in others, the knowledge of the effect usually precedes the knowledge of the cause.

An animal wandering in a strange territory follows on his return the reverse of the road, more or less winding, by which he came. When he reaches known territory he moves in a straight line to his destination.

The carrier pigeon, set at liberty at a distance of some 500 kilometers from its home, follows, in returning, the railroad which brought it; it is now guided by its sixth sense. Having in this way reached the known horizon, say 80 kilometers from its home, it no longer depends on its sixth sense, but goes by its sight straight homeward.

At other times when it reaches known regions the pigeon does not think of making use of its five senses, but follows its former path back to its cote. Sometimes it goes past it; thus we have seen pigeons returning from a long journey pass within 40 or 50 meters of the cote, go on and only return after an hour or two, having covered in this way, perhaps from 30 to 60 kilometers in the wrong direction.

If a common pigeon, accustomed to using almost exclusively its five senses, and a carrier pigeon broken to long voyages, are carried about 10 kilometers away from the cote, when they are successively released an interesting fact is noticeable—the ordinary pigeon, going by sight, will usually make its way much more rapidly than the carrier pigeon, who will find its way back carefully with the aid of its sense of direction.

From this fact we may conclude that the sense of direction does not combine its action with that of the live others. It begins to act in a zone where the other senses are inactive, and often continues to act in the known region to the exclusion of the other five senses.

It seems that it is not actuated by impressions received from the path followed and that it is in some degree a subjective organ. We