22 PERIOD I. 



pricked a liquid resembling' milk gushed out. Further 

 examination showed him that these vessels, like the 

 veins, possess valves which permit flow in one direction 

 only. Pecquet, a French physician, announced in 1651 

 that the lacteals open into a thoracic duct, which joins 

 the venous system. In 1653 Rudbeck of Upsala 

 described yet another set of vessels, the lymphatics ; 

 these again are provided with valves, and open 

 into the thoracic duct, but are filled with a clear 

 liquid. 



The effect of these discoveries upon physiology and 

 medicine was very great, but it did not end there ; the 

 whole circle of biological students and a still wider 

 circle of men who pursued other sciences were thereby 

 encouraged to followthe experimental path to knowledge. 

 Wallis, in describing the meetings of scientific men held 

 in London in 1645 and following years, mentions the 

 circulation of the blood, the valves in the veins, the 

 lacteals, and the lymphatic vessels among the subjects 

 which had stirred their curiosity ; while the naturalist 

 Ray thanked God for permitting him to see the vain 

 philosophy which had pervaded the University in his 

 youth replaced by a new philosophy based upon experi- 

 ment a philosophy which had established the weight 

 and spring of the air, invented the telescope and the 

 microscope, and demonstrated the circulation of the 

 blood, the lacteals, and the thoracic duct. 



The Natural History of Distant Lands (Sixteenth 

 Century and Earlier). 



Travel and commerce had made the ancient world 

 familiar with many products of distant countries. Well- 

 established trade routes kept Europe in communication 

 with Arabia, the Persian Gulf, and India. Egyptians, 



