CHAPTER VIII. 

 THE BLOOD. 



74. HAVING traced the process of digestion, it would be 

 natural to pursue the history of the new supplies of nourish- 

 ment after their entrance into the economy from the alimentary 

 tube. It will be found, however, to be more convenient, if, 

 instead of adhering strictly to the course taken by these 

 supplies, we first consider the blood, and afterwards the 

 streams which fall into it, and its mode of elaboration. 



On account of the extreme facility with which substances 

 pass outwards and inwards between the minutest vessels 

 and the tissues, and the impossibility of completely emptying 

 the vascular system, even in the bodies of animals, it is 

 exceedingly difficult to estimate the amount of the blood. 

 In one observation, in which the blood was carefully washed 

 from the bodies of two executed criminals, and the calcula- 

 tion based on the amount of solid matter obtained, the 

 weight of blood was estimated as one-eighth of that of the 

 body (Weber and Lehmarm). According to other calcula- 

 tions founded on observations 011 animals, and made by 

 mixing a portion of blood with a known amount of water, 

 then washing out the vessels, and reducing the washings to 

 the same tint as the standard solution, it was computed at 

 about one-thirteenth of the weight of the body, or twelve 

 pounds in a person eleven stones weight (Welcker). 



75. When blood flows from a wound it speedily coagulates 

 or runs into a clot. This depends on the presence of .a 

 spontaneously coagulable albuminoid substance, called fibrin, 

 which, being diffused through the blood, entangles the other 

 constituents in its meshes. But if the blood be allowed to 

 remain in a vessel, the coagulum contracts, and expels from 

 it the serum, a straw-coloured fluid which may be more or 



