IN MEDICINE. 17 



the study of illness, it is necessary for us to understand the 

 minute anatomy of every organ and the mechanism by which the 

 various physiological actions are performed in health. 



Now, careful dissection will reveal to us much of the structure 

 of the body as far as it can be seen by the unaided eye, but 

 beyond that limit it is impossible to carry our investigation without 

 the aid of the microscope, while by it, in every part of the 

 organism, we are now able to trace structures which were before 

 unknown. Take, for instance, the arrangements for the circula- 

 tion of the blood. Harvey had shown that the blood, distributed 

 by the left side of the heart through the general arteries of the 

 body, returned through the veins to the right side, and thence, 

 after passing through the lungs, again reached the left side ; but 

 how the blood passed from the minutest arteries to the earHest 

 veins, the older physiologists were not a little puzzled to understand. 

 They traced these vessels as far as they could see them, and 

 when they could no longer do so, they supposed they must have 

 terminated in small, open mouths, by means of which the blood 

 was emptied into the tissues, which then helped themselves to 

 what they wanted, and passed on the remainder into the open 

 mouths of the minutest veins, for it to be returned to the heart. 



But now, with the help of the microscope, we can follow the 

 whole course of the circulation in the web of the frog's foot, and 

 see that the smallest arteries lose themselves in a network of fine 

 vessels called Capillaries, out of which, as out of so many roots, a 

 small vein gathers itself together again. These small vessels are 

 seen to be minute channels of uniform size, their average diameter 

 being about one-three-thousandth of an inch, composed of 

 extremely fine, transparent walls, and spread over every part of 

 the body, and that so closely, that it is impossible to prick oneself 

 in any part without rupturing some of them. These minute 

 vessels carry to every organ the nutritive fluid without which 

 life and growth could not be supported, and they also carry the 

 blood to various organs, in which it is purified and regenerated, 

 and made fit for supporting the vital functions, their manner 

 of distribution being modified according to the requirements 

 of each part. 



Thus, in the lungs, we see by the microscope, that the ultimate 



