4 OUTLINES OF NATURAL HISTORY. 



stantly draining away from it nutritive materials, and it 

 has therefore to be constantly receiving a supply of these 

 same materials from the food, as prepared by the digestive 

 organs. In the second place, the various tissues of the 

 body are constantly throwing off into the blood waste 

 matters, and the blood must get rid of these, if it is to 

 retain its purity. This latter process is generally effected 

 by means of distinct breathing organs, in which the blood 

 is exposed to the action of the air. In the air-breathing 

 animals the breathing organs are filled with air directly ; 

 in the water-breathing animals the breathing organs are 

 adapted for absorbing the air which is generally dissolved 

 in water. In either case, the waste materials contained in 

 the blood are got rid of by the action of the oxygen con- 

 tained in the air, which unites with them, and literally 

 hxirns them up. 



The main function, then, which any animal has to per- 

 form, is to no^irish itself, or, in other words, to supply itself 

 with materials capable of taking the place of the tissues 

 which become worn out in the discharge of the vital func- 

 tions. Animals, however, have more to do than this. 

 They have to become acquainted with what is outside 

 themselves, and to hold certain relations with the external 

 world ; and these relations come in a general way under 

 the heads of "locomotion" and "sensation." Most ani- 

 mals, namely, can change their relations to external ob- 

 jects, as regards space, by moving, and their movements 

 are generally effected by means of special locomotive 

 organs. Most animals possess the power of changing 

 their place bodily, and even those which cannot do this, 

 by reason of their being permanently rooted to one place, 

 can nevertheless move their bodies more or less freely. 

 The organs of locomotion, or rather the agents of loco- 

 motion, are usually "muscles;" but some animals move 

 about without having any distinct muscles, or indeed any 

 permanent locomotive organs. In most cases, also, the 

 organs of locomotion are brought under the control of 

 the animal's will by means of the special structures which 

 constitute a "nervous system." It is not, however, by 



