202 OF THE BLOOD, AND ITS CIRCULATION. 



disappears ( 203), the function of absorption is not in any 

 way limited ; since every part seems to have the power of re- 

 ceiving from without, and of assimilating to its own substance, 

 the nutrient materials which it needs. 



CHAPTER V. 



OF THE BLOOD, AND ITS CIRCULATION. 



226. The processes that have been already explained, have 

 for their object to prepare the nutritious fluid, which supplies 

 the materials for the growth of the several parts of the body, 

 and which is conveyed through them by the apparatus to be 

 presently described. In Man and the higher animals, this 

 fluid, which is known as the Blood, has a red colour, and con- 

 tains a large quantity of solid matter. The redness of the blood 

 has been mentioned as a distinctive character of the Yerte- 

 brated classes ( 75) ; it exists in Mammalia, Birds, Eeptiles, 

 and Fishes, and in these alone. In the Molluscous classes, as 

 also in most of the Articulated, the nutritious fluid is nearly 

 colourless ; and it will hereafter appear that this fluid bears, 

 in some respects, a stronger resemblance to the chyle and 

 lymph of the Yertebrata, than to their blood ( 234). There is 

 an apparent exception in the case of certain marine "Worms, 

 the fluid circulating in whose vessels has a reddish hue ; this 

 does not depend, however, upon the presence of any red par- 

 ticles, such as are characteristic of the blood of Yertebrata 

 ( 229), but upon a reddish tinge in the fluid itself, which 

 does not seem altogether to answer to the character of 

 blood ( 294). 



227. The blood of all the higher animals exists in two 

 different states. When it is drawn from a slight scratch or 

 other wound of the skin, it is of a bright red hue ; whilst that 

 which is drawn in bleeding from the arm, is of a dark purple. 

 The former is termed arterial blood, because it is contained, for 

 the most part, in the tubes which are called Arteries, and 

 which are conveying it from the heart to the tissues it has to 

 nourish. The latter is called venous blood, because it is drawn 

 from the Yeins, by which it is returned from the tissues to 

 the heart, after having performed its part in them. Hence it 



