no RESPIRATION 



Cold-blooded animals include all the Invertebrata and such 

 Vertebrata as the fishes, amphibia, and reptiles, but even 

 these have as a rule a temperature a few degrees above that 

 of the external surroundings. The viper has a temperature 

 of 68° F. to 84° F., the frog of 58° F. to 63° F., the carp 69° F. 

 and leeches 57° F. Although a single bee becomes numb and 

 inert at the temperature of a summer night, the vast numbers 

 of individuals in a hive produce a considerable amount of 

 heat, enough to keep the bees active during the winter. In 

 December at a time when the external air was only 42° F. 

 the temperature in the interior of a hive has been found to 

 be 84° F.; and this is indeed necessary, as the larvae, pupae, 

 and eggs all died in a temperature below 33° F. It is also 

 necessary in order to keep the wax soft and malleable. 



The tissues of an animal are constantly discharging carbon 

 dioxide into the blood and picking up oxygen from it. Both 

 the corpuscles and the plasma contain carbon dioxide, and 

 blood contains about the same proportion of carbon dioxide 

 as water does when it has been shaken up in the presence of 

 that gas. In mammals blood is driven from the right side of 

 the heart to the lungs. Here it frees itself of its carbon dioxide 

 and picks up oxygen. The arterial blood, as it is then called, 

 returns to the left side of the heart and is driven round the 

 body, where it readily yields up oxygen to the oxygen-hungry 

 cells, and picks up carbon dioxide before regaining the right 

 side of the heart. It passes from the arteries through the 

 capillaries to the veins and so back to the heart. 



The number of capillaries is enormous, though that of 

 those that are open at the same time in a muscle varies 

 according to its state of activity. The section of an ordinary 

 pin is about half a square millimetre, yet in a square millimetre 

 of the muscles of a horse there are 3150 capillaries. In smaller 

 animals such as the guinea-pig the number per square milli- 

 metre is even higher. This means that a very large surface of 

 blood is available for interchange to take place with the cells 

 of the tissue. Krogh has made the following calculation: 

 "Supposing a man's muscles to weigh 50 kilograms and his 

 capillaries to number 2000 per square millimetre, the total 

 length of all these tubes put together must be something like 



