20 THE FUNCTIONS OF ANIMALS. 



kidneys not only cast out of the blood all the waste-products 

 that result from the metabolism of proteids and contain 

 nitrogen ; but they maintain the composition of the blood at 

 its normal, rejecting any stuffs that vary from that norrnal, 

 l^ither qualitatively or quantitatively, doing this work according 

 to laws quite different from the simple ones of diffusion or 

 solubility: thus, sugar and urea are about equally soluble, 

 and yet the sugar is kept in the body, while the urea is 

 cast out. Even substances as insoluble as resins are removed 

 from the blood by the living cells of the kidneys. 



A considerable quantity of water, and traces of salts, fats, 

 etc., leave the body by the skin, but its chief use is to pro- 

 |fect, and to regulate the temperature by variations in the 

 %ize of its blood-vessels. 



This completes our sketch of the process by which the 

 food becomes available for the organism as fuel for the 

 maintenance of its life energies, and of the removal of the 

 waste-products which are formed as the ashes of its life. The 

 purpose of the preceding sketch is merely to give a general 

 knowledge of the functions of those organs which will be 

 discovered in the dissection of animal types, and of which 

 the student beginning the study of Zoology is generally 

 ignorant. Little need be said, therefore, of the muscles, for 

 every one knows that by contracting they produce move- 

 ments of the body or of its parts. The contraction of a 

 muscle-cell or fibre involves, (i) a visible change of form, (2) 

 the liberation of heat, and the discharge of water and 

 /tjcarbon dioxide, which show that a chemical change takes 

 Ifplace within, and (3) certain subtle electrical changes, the 

 meaning of which we do not clearly understand. As to the 

 rapid chemical change, the results of which are known, it is 

 plausible to suppose that there is within the muscle-fibre and 

 under the regulation of its living matter, an encounter 

 between oxygen on the one hand, and some readily oxidisable 

 material on the other. The chemical energy of this en- 

 counter is transformed into heat and into the kinetic energy 

 bf muscle-movement by which work is done. 



Nor shall we discuss the nervous system, because to treat 

 it even in the simplest way would occupy a great deal of 

 space, and because every one knows in a general way that 

 ganglia or collections of ganglia, such as our brain, are 



