CHAPTER V. 



THE BLOOD. 



GENERAL POINTS. 



When an incision is made into a living body there at 

 once streams out a reddish-looking liquid familiar to every 

 one as blood. If this substance, which seems at first sight 

 nothing but a liquid, is examined under a lens, it is found 

 that the liquid itself is not red, but that its color is due to 

 little particles which are colored red, suspended in it." These 

 little particles are the red corpuscles of the blood, whose 

 coloring is due to an iron compound called haemoglobin. 

 So accustomed have we become to the association of red- 

 ness with blood that one feels tempted at first view to deny 

 the existence of blood to those forms whose circulating 

 liquid is not colored red. But the blood of most of the 

 invertebrates is colorless and devoid of this pigment. 



The necessity for blood is too evident to need comment. 

 Tissues in various parts of the body must have nourish- 

 ment brought to them and must have their wastes removed, 

 and these results can be obtained only by having a circu- 

 lating medium which shall answer to this purpose. Some 

 of the few-celled animals so small that the juices may 

 reach all parts of their bodies by the mere process of 

 osmosis, possess no real blood at all. Most of the lower 

 forms are, however, provided with a circulating fluid quite 

 similar to the blood of the vertebrates, except that it con- 

 tains no red corpuscles. The colorless blood of the clam 

 and oyster are matters of common observation. In some 

 of these invertebrates the liquid itself is colored red with 

 haemoglobin, as for instance, in the earth worm, whose 

 reddish blood vessels are easily seen through the transpar- 

 ent skin, while in certain exceptional forms actual colored 

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