92 THE SCAFFOLDING LEFT IN THE BODY. 



the whole scalp to and fro, and the muscular apparatus 

 for effecting it is identical with what is normally found 

 in some of the Quadrumana. 



Another typical vestigial structure is the plica 

 semi-lunaris, the remnant of the nictitating mem- 

 brane characteristic of nearly the whole vertebrate 

 sub-kingdom. This membrane is a semi-transparent 

 curtain which can be drawn rapidly across the ex- 

 ternal surface of the eye for the purpose of sweeping 

 it clean. In birds it is extremely common, but it also 

 exists in fish, mammals, and all the other vertebrates. 

 Where it is not found of any functional value it is 

 almost always represented by vestiges of some kind. 

 In Man all that is left of it is a little piece of the 

 curtain draped at the side of the eye. 



Passing from the head to the other extremity of the 

 body one comes upon a somewhat unexpected but 

 very pronounced characteristic the relic of the tail, 

 and not only of the tail, but of muscles for wagging it. 

 Every one who first sees a human skeleton is amazed 

 at this discovery. At the end of the vertebral column, 

 curling faintly outward in suggestive fashion, are 

 three, four, and occasionally five vertebrae forming the 

 coccyx, a true rudimentary tail. In the adult this is 

 always concealed beneath the skin, but in the embryo, 

 both in Man and ape, at an early stage it is much 

 longer than the limbs. What is decisive as to its true 

 nature, however, is that even in the emb^o of Man 

 the muscles for wagging it are still found. In the 

 grown-up human being these muscles are represented 

 by bands of fibrous tissue, but cases are known where 

 the actual muscles persist through life. That a dis- 

 tinct external tail should not still be found in Man 



