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 Y. 
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 Verte- 
brated classes (§7 5); it exists in Mammalia, Birds, Reptiles, 
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 Vertebrata, 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 Vertebrata 
(§ 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 Veins, by which it is returned from the tissues to 
the heart, after having performed its part in them. Hence it 
