INTRODUCTION 7 



An animal ingests bulky food in a crude state ; before this 

 food can be made serviceable, and can be converted into 

 actual living matter, or stored up in some form in which it 

 may be serviceable hereafter (e.g. in the form of fat), it has 

 to undergo a process of chemical elaboration. The great 

 majority of animals have a special aperture for taking in food, 

 the mouth. This is very commonly furnished with special 

 mechanisms for seizing and comminuting the food viz. with 

 jaws and teeth. The mouth leads into a more or less 

 capacious tube, the gut, in which the process of elaboration 

 goes on. The gut is a sort of laboratory, and is provided with 

 the necessary stock of chemical solvents poured into it from 

 special receptacles, the glands, with which it is abundantly 

 provided. The substances poured in by the glands are known 

 as secretions : they act chemically upon the food, rendering it 

 soluble and capable of passing through the relatively thin walls 

 of which the gut is composed. Those parts of the food which 

 are insoluble are expelled, usually through a special aperture, 

 the vent or anus. The food, altered and rendered soluble, 

 passes through the walls of the gut. It has to be distributed 

 to all parts of the body many being situate at some distance 

 from the gut. To this end we find a mechanism for distribu- 

 ting the nutriment to all parts, commonly in the form of a 

 system of tubes, containing a fluid in which certain solids are 

 suspended. There are,two such fluids in ourselves, the blood 

 and the lymph ; each with its proper system of channels and 

 vessels. By the blood and the lymph the nutriment is con- 

 veyed to every tissue, and in the tissues themselves it under- 

 goes those further changes which may be described as trans- 

 formation into living substance of unstable composition and 

 high potential energy. The blood is kept in movement by 

 the contractions of a hollow muscular sac, the heart. The 

 blood, however, is much more than a carrier of nutriment. 

 The complex living substance liberates energy in undergoing 

 further chemical change, on breaking down again into simpler 

 substances, and this change is at the bottom a process of 

 oxidation. A supply of oxygen is therefore necessary in every 

 part of the body, and this is provided for by a special respira- 

 tory mechanism. There are various kinds of respiratory 

 mechanisms, but they are all alike in this respect, that they 

 afford a means of interchange between the gases dissolved in 



