CHAPTEE I. 

 BLOOD. 



BLOOD, when flowing in a normal condition through the blood- 

 vessels, consists of an almost colourless fluid, the plasma, in which 

 are suspended a number of more solid bodies, the red and white o ' 

 corpuscles. Were we anxious to give a formal completeness to the 

 classification of the various parts of the body into tissues, we might 

 speak of the blood as a tissue of which the corpuscles are the 

 essential cellular elements, while the plasma is a liquid matrix. 

 We might compare it to a cartilage, the firm matrix of which had 

 become completely liquefied so that the cartilage-corpuscles were 

 perfectly free to move about. 



In regarding blood as tissue, however, we come upon the 

 difficulty that it, unlike all the other tissues, possesses no one 

 characteristic property. The protoplasm of the white corpuscles 

 is native un differentiated protoplasm, in no respect fitted for any 

 special duty ; and though, as we shall see, the red corpuscles have 

 a definite respiratory function, inasmuch as they are carriers of 

 oxygen from the lungs to the several tissues, still this respiratory 

 work is only one of the very many labours of the blood. It will 

 be therefore far more profitable, indeed necessary, to treat of the 

 blood, not as a tissue by itself, but_as the great means of com- 

 munication of material between the tissues properly so called. Its 

 real usefulness lies not so much in any one property of either its 

 corpuscles or its plasma, as in its nature fitting it to serve as the 

 great medium of exchange between all parts of the body. The 

 receptive tissues pour into it the material which they have received 

 from without, the excreting tissues withdraw from it the things 

 which are no longer of any use, and the irritable, the contractile, 

 and indeed all the tissues, seek in it the substances (including 



