THE DEVELOPMENT OF REFLEXES 



285 



Fig. 189. The human fetus about four 

 months old. (From Julius Kollman, Hand- 

 atlas der Entivickhoigsgeschicbte des Men- 

 scben. Verlag von Gustav Fischer, Jena.) 



mechanical, chemical, or thermal stimulation. Then, too, studies on other 

 mammalian fetuses bring out the point that the fetal neuromuscular system 

 has a much higher threshold for stimulation than the adult. 



By the sixteenth week certainly, and probably as early as the twelfth 

 week, the swallowing reflex begins by muscular contraction. This leads to 

 swallowing of the amniotic fluid in the older fetus. The proof that the fetus 

 "drinks" its own amniotic fluid comes from the results of experiments in 

 which colloidal thorium dioxide is injected into the amniotic fluid at about 

 five months. After about 15 hours the uterus is removed, and a photograph 

 of the fetus taken with X rays shows the colloidal thorium in the stomach 

 and intestine. This knowledge has been used in an interesting treatment ad- 

 ministered to mothers having too much amniotic fluid. By the injection 



