162 THE MINUTE ANATOMISTS 



out lung became sufficiently transparent to reveal a 

 network of fibres or vessels binding together the cells. 

 When water was injected into the pulmonary artery, a 

 stream issued from the pulmonary vein. How, he asked, 

 do the branches of the artery end ; by anastomosis with 

 the vein, or by open mouths, leading into air-filled 

 spaces, as most physiologists then held ? Before seeking 

 an answer to this question, he wrote an account of his 

 preliminary inquiries to his master and friend Borelli ; 

 this letter is the first of two on the lungs, which were 

 printed and became famous. He then betook himself 

 to close study of the lung of the frog, which is both 

 unusually transparent and of comparatively simple 

 structure. 



In January 1661, only a few days, it would seem, 

 after despatching his first letter, Malpighi had important 

 news for Borelli. He had found in the frog's lung air- 

 cells of simple form, enclosed by folds of the lining 

 membrane, and subdivided by smaller folds. Upon the 

 edges of all the folds ran blood-vessels of various size.^ 

 In a lung, still connected with the beating heart, he 

 had seen the blood-stream coursing through finer and 

 finer branchfes, and becoming paler as it broke up. 

 Then the branches seemed to reunite into a vein, but 

 Malpighi was not certain of the fact until he examined 

 by the microscope a dried frog's lung, in which the 

 finest vessels were naturally injected with blood. It 

 then became clear to him that the blood never escapes 

 from the vessels, but passes from artery to vein through 

 a closed capillary network, and so returns by the 

 pulmonary vein to the heart. The irregular cavities, 

 into which, according to Fabricius, the pulmonary 



' Malpighi's figures show that he did not perfectly understand the details 

 of structure, and his description has been amended here. 



