I] IlSrTRODUCTION 7 



the heart alternately contracting and dilating, the compressing 

 force being supplied by the muscular wall of the organ. The" 

 heart communicates with an elaborate system of vessels, those 

 which carry the blood away from it being called arteries, while 

 those which bring the blood back are called veins. The arteries 

 possess thicker and more elastic walls than the veins. The 

 arteries become divided up in the peripheral parts of the body 

 into a number of much smaller vessels, the capillaries, which 

 permeate the tissues. Some of the fluid of the blood transudes 

 through the walls of these vessels, becoming the lymph and 

 bathes the cells with blood. The capillaries unite again to form 

 the veins, which transport the blood back to the heart. If the 

 web of a living frog's foot be examined under the microscope, 

 the circulation of the blood in the capillaries may be observed 

 quite easil}'. 



The red colour of blood is due to the presence of innumerable 

 little red disks known as corpuscles, which float in a yellowish- 

 coloured semi-transparent fluid, the blood plasma. In addition 

 to these red corpuscles, blood also contains a relatively small 

 number of white corpuscles or leucocytes, which have the power 

 of independent movement and resemble the unicellular amoebae 

 referred to above. One of their functions is to eat up and so 

 destroy the germs of disease. 



As already mentioned some of the products of digestion are 

 absorbed into vessels containing lymph which is a colourless fluid 

 resembling blood plasma. The lymph vessels or lymphatics com- 

 municate with veins so that the substances present in h^mph 

 eventually enter the blood system. 



Respiratory System. In order to keep an animal alive it is 

 necessary to su))ply it with ox3'gen, for, as we have seen, every 

 animal is dependent for its source of energy upon the slow oxida- 

 tion of the material which it builds up out of its food supply. 

 As a result of this oxidation process it is continually giving off 

 carbon dioxide. In the higher vertebrates the organs which are 

 concerned in this gaseous exchange are the lungs ; but in the 

 lower forms of life the entire surfaces of the bodj' serve to effect 

 the interchange of oxygen and carbon dioxide ; while in many 

 other animals, which live in water (e.g. fishes), the respiratory 

 organs take the form of gills. 



The lungs of an animal are connected with the exterior by 



