THE CIRCULATION OP THE BLOOD. 267 



Fig. 6. 





A portion of fibrin, showing its fibrous structure and the net-like 

 arrangement of its fibres. 



pic observations show that they are flattened cakes, having 

 rounded and slightly thickened margins, as shown in the 

 following cut : 



Fig. 7. 



1. A particle of human blood as it appears when transparent and 

 floating. 2. The same, seen as illuminated. 3. The same, one half 

 illuminated. 4. A particle of frog's blood floating. 5. The same, 

 seen edge-wavs. All these objects are magnified five hundred diam- 

 eters. 



28. (The blood constitutes about one fifth part of the 

 weight of the whole bodyj; so that the whole quantity of 

 blood in the body of an adult may be estimated at between 

 thirty and forty pounds. ^A complete revolution of the 

 blood takes place every three minutes, and there are about 

 five hundred and fifty revolutions every twenty-four hours/ 

 The contractions of the ventricles, or the pulse, is about 

 seventy-five times in a minute, in a grown person, and one 

 hundred and forty in an infant ; in old age about sixty. 



