438 THE EVOLUTION OF MAN. 



skin we also have entirely unused skin-muscles ; vestiges of 

 the largely developed skin-muscles of our lower mam- 

 malian ancestors. It was the function of this " panniculus 

 carnosus " to contract and wrinkle the skin, as we may see 

 any day done by horses to drive away flies. We still 

 possess an active remnant of this great skin-muscle in the 

 muscle of the forehead, by means of which we wrinkle the 

 forehead and draw up the eyebrows ; but we are no longer 

 able to move at will another considerable remnant of it, the 

 great skin-muscle of the neck (platysma myoides). 



As in these animal organ-systems of our body, so also in 

 the vegetative apparatus, we meet with many rudimentary 

 organs, most of which we have incidentally noticed. I will 

 only cite the remarkable thyroid gland (thyreoidea), the 

 rudiment of the crop and the remnant of the ciliated groove 

 (hypobranchial groove) present in Chordonia, Ascidia, and 

 Arcrania, on the lower part of the gill-body (pp. 336, 353) ; 

 also the vermiform process of the blind-intestine (caecum) 

 (p. 344). In the vascular system we find many useless 

 ducts, the vestiges of disused vessels which were formerly 

 active blood-channels ; such, for instance, are the " ductus 

 Botalli," between the lung-artery and the aorta, and 

 the "ductus venosus Arantii," between the vena portce 

 and vena cava, and many others. The numerous rudi- 

 mentary organs of the urinary and sexual systems (p. 415) 

 are especially interesting. Most of these are developed in 

 one sex and rudimentary in the other. Thus, in the male, 

 the seed-ducts form from the Wolffian ducts, of which the 

 only traces remaining in the female are the Gartnerian 

 canals. On the other hand, from the Mullerian ducts in the 

 female are developed the oviducts and the uterus ; while in 



