304 The Outline of Science 



He is memorable not only for his anatomy of small creatures, but, 

 like Malpighi, for his minute anatomy of larger ones, and here we 

 might also include the early British microscopists, Hooke and 

 Grew. For this was another line of advance, to disclose the in- 

 tricacy of vital architecture that lay beyond the limits of scalpel 

 and simple lens. Thus it was a great step when Swammerdam dis- 

 covered in 1658 the blood corpuscles of the frog; when Malpighi 

 demonstrated the air-cells in the lung where the gaseous inter- 

 change takes place between blood and air; when Leeuwenhoek 

 completed Harvey's theory of the circulation of the blood by 

 demonstrating in 1680 the capillary connection between arteries 

 and veins. Speaking of the tail of the tadpole, he said, "A sight 

 presented itself more delightful than any mine eyes had ever be- 

 held ; for here I discovered more than fifty circulations of the blood 

 in different places, while the animal lay quiet in the water, and I 

 could bring it before my microscope to my wish. For I saw not 

 only that in many places the blood was conveyed through exceed- 

 ingly minute vessels, from the middle of the tail toward the edges, 

 but that each of the vessels had a curve or turning, and carried the 

 blood back toward the middle of the tail, in order to be again con- 

 veyed to the heart." Such was the momentous observation of the 

 fact that the arteries leading from the heart, and the veins leading 

 back to the heart are bound into one system by the intermediation 

 of the capillaries. 



This is an easy illustration of the kind of service microscopy 

 has never ceased to render making vital activity more intelligible 

 by revealing the intricacy of structure. For it is in a study of the 

 structure that we get a better understanding of the ways and 

 means of life. It is not the whole story of the workshop to know 

 the furnishings and the tools, but it is an essential part of the story. 

 We hastily draw away our finger from a hot plate a reflex action 

 it is only with the help of the microscope that the physiologist 

 can tell how the message travels by sensory nerve-cells to inter- 

 mediary nerve-cells and thence to motor nerve-cells which com- 



