VESTIGIAL ORGANS
muscles direct the ear as an ear-trumpet might be turned in relation to a source of sound, and the muscles of the ear proper change slightly the shape of that organ to adapt it better, perhaps, for the reception of a given sound. The human ear is incapable of these movements. The muscles, though present, are ordinarily functionless, though occasionally a person will be found who can move his ears slightly and in this way demonstrate a limited control over some of these muscles, but even the movement he produces is so slight as to have no advantage whatever for hearing and to be rather a lusus naturae than an
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act of physiological importance. In the horse and the dog the movements of the ears are of great value in discovering the direction of sound, but the muscles of the external ear of man perform no work comparable to that of the ears of lower animals. In man the muscles of the external ear are, in the strictest sense of the word, vestigial.
Not only does the ear of man exhibit vestigial organs but a similar organ is found in the eye. Deep-seated in the nasal angle of the eye of man is a crescentic ridge of whitish tissue which, in consequence of its shape, is called the plica semilunaris or semilunar fold. It is not an organ that plays an important part in the action of the eye; in fact, it appears to be little more than a mechanical duplicature in the membrane in adjustment to the surface which it covers, and no one would suspect its meaning until he had examined the eyes of lower animals.
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