450 HISTORY OF PHYSIOLOGY. 



which are connected with them, were further illus- 

 trated by Ruysch in his exhibition of their valves 

 (DUuddatio valvularum in vasis lymphaticis et 

 lacteis). 



Sect. 2. The Consequent Speculations. Hypo- 

 theses of Digestion. 



THUS it was shown that aliments taken into the 

 stomach are, by its action, made to produce chyme; 

 from the chyme, gradually changed in its progress 

 through the intestines, chyle is absorbed by the 

 lacteals; and this, poured into the blood by the 

 thoracic duct, repairs the waste and nourishes the 

 growth of the animal. But by what powers is the 

 food made to undergo these transformations ? Can 

 we explain them on mechanical or on chemical 

 principles ? Here we come to a part of physiology 

 less certain than the discovery of vessels, or of the 

 motion of fluids. We have a number of opinions 

 on the subject, but no universally acknowledged 

 truth. We have a collection of Hypotheses of Di- 

 gestion and Nutrition. 



I shall confine myself to the former class ; and 

 without dwelling long upon these, I shall mention 

 some of them. The philosophers of the Academy 

 del Cimento, and several others, having experimented 

 on the stomach of gallinaceous birds, and observed 

 the astonishing force with which it breaks and 

 grinds substances, were led to consider the digestion 



