HEREDITY 



175 



Russian dog man, showing extraor- 

 dinary covering of hair on the face. 

 (After Wiedersheim.) 



developed in the embryo than in 

 the adult, becoming atrophied 

 with age. Familiar examples 

 are the appendix vermiformis 

 and the unused muscles of the 

 ears in man, the atrophied lung, 

 pelvis, and limbs of the snake, 

 the air bladder of the fish, the 

 "thumb" (or rather index fin- 

 ger), of the bird, the splint bone 

 of the horse, and the like. 



The anatomist Wiedersheim 

 has recorded 180 vestigial or- 

 gans in man. These structures 



OCCUr in all the Systems of Fl - 108. Andrian Jeftichjew, the 



organs, integument, skeleton, 

 muscles, nervous system, sense 

 organs, digestive, respiratory, 



circulatory, and urino-genital systems. Most of these rem- 

 nants of structures are to be found completely developed in 

 other vertebrate groups. Eleven of them are characteristic 

 as functional organs of fishes only, four of amphibians and 

 reptiles. The fact that structures are vestigial is shown often 



by cases of atavistic de- 

 velopment. 



Within the brain of 

 man, near the optic lobes, 

 is a little spheroid structure 

 scarcely larger than a pea, 

 known as the "pineal 

 gland" or conarium. It 

 has no evident function, 

 and Descartes once sug- 

 gested that it might be the 

 seat of the soul. It is 

 larger in the embryo and 

 still larger in the brains of 

 some of the lower verte- 

 brates. Recent investiga- 

 tions have shown that it 



FIG. 109. Pineal eye of lizard, Hatteria. . . -,-, -, , j 



(After spencer.) is especially developed in 



