^ BLOOD.] PHYSIOLOGY, 41 



you see are floating in a fluid so clear that yon 

 cannot see it. Some of the smaller channels are 

 so narrow that only one globule or corpuscle, 

 as we may call it, can pass through at a time, 

 and very frequently you may see them passing in 

 single file. Watching them as they glide along 

 these narrow paths, you will note that at last they 

 tumble again into wider passages, somewhat like those 

 from which they came, except that the stream runs 

 away from instead of towards the narrower channels ; 

 and in the stream the corpuscle you are watching 

 shoots out of sight. The finest passages are called 

 capillaries ; they are guarded by delicate walls 

 which you can hardly see ; they seem to you passages 

 only, and how fine and small they are will come 

 home to you when you recollect that all you are 

 looking at is going on in the depths of a skin which 

 is so thin that perhaps you would be inclined to say 

 it has no thickness at all. 



The larger channels which are bringing the blood 

 down to the capillaries are the ends of vessels like 

 those which in the rabbit you learnt to call arteries, 

 and the other larger channels through which the 

 blood is rushing away from the capillaries are the 

 beginnings of veins. 



When you hav€i watched this frog's foot for some 

 little time, turn away and reflect that in almost every 

 part of your own body, in every square inch, in 

 almost every square line, something very similar 

 might be seen could the microscope be brought to 

 bear upon it, only the corpuscles are smaller and 

 round, the capillaries narrower and for the most 

 part more thick-set, and the race a swifter one. In 



