ioo FATHERS OF BIOLOGY. 



the latter, through the vaso motor system, regulating the 

 flow of blood to particular parts in order to meet 

 changing requirements. 



It is somewhat surprising to find that such an accurate 

 observer as Harvey should have failed to recognize the 

 significance and importance of the system of lacteal 

 vessels. But such was the case. Eustachius, in the 

 sixteenth century, had discovered the thoracic duct in 

 the horse, although he seems to have thought that it 

 was peculiar to that animal. Aselli, while dissecting the 

 body of a dog in 1622, accidentally discovered the 

 lacteals, and thought at first that they were nerves ; but 

 upon puncturing one of them, and seeing the milky fluid 

 which escaped, found them to be vessels. He, however, 

 failed to trace them to the thoracic duct, and believed 

 them to terminate in the liver. Pecquet of Dieppe 

 followed them from the intestines to the mesenteric 

 glands, and from these into a common sac or reservoir, 

 which he designated receptaculum chyli, and thence to 

 their entry by a single slender conduit into the venous 

 system at the junction of the jugular and subclavian 

 veins. The existence of the lacteals had not entirely 

 escaped Harvey, however. He had himself noticed them 

 in the course of his dissections before Aselli's book was 

 published, but " for various reasons " could not bring 

 himself to believe that they contained chyle. The small- 

 ness of the thoracic duct seemed to him a difficulty, and 



