THE activities of an animal are such that in many respects we may liken it to a piece of machinery, such as a motor car, which has been set working by man, and it is instructive to compare the working of the man-made machine with a living one. The motor car is made to work by the explosion caused by an electric spark, which occurs in the cylinders. Energy is set free from an explosive mixture of petrol vapour and air. This energy in the form of heat causes the gases present to expand so setting up a pressure in the cylinders sufficient to push down the pistons. These transmit their motion by the connecting rod to the crankshaft, thus causing it to rotate. The fundamental feature of this is the combustion of petrol vapour, thereby liberating energy stored in the petrol. All the mechanical activities carried out by the animal also require energy. The mechanisms, I.e. the working parts, are the muscles, and these carry out work in an animal such as man, a rabbit or a frog by changing from a shape which is fairly long and thin to one which is shorter and fatter. The fuel in this case is not petrol vapour but compounds—nearly always simple sugars—which are broken down, thereby setting free useful energy which enables the muscles to contract, and the movement of parts results. In the motor car the supplies of air and petrol are ’fed ’to the cylinders by the carburettor. In the animal the muscles are supplied with the ’fuel ’by the blood stream. Thus in all cases the materials are carried to the places where they will be used, in a form which is easily supplied, to set free the energy necessary to do the work. In an animal the process by which the useful energy is set free is called tissue respiration.
A difference now arises between the two cases in that the combustible material in the car is already refined, having been previously obtained from the highly complex mixture known as crude petroleum from oil wells. The food of an animal, however, consists of various compounds which have to be broken down by the animal itself into more simple soluble ones, so that they can be carried by the blood to the muscles. The alimentary canal carries out this simplifying process, and when the useful materials have been carried away, the useless residues are voided. This is somewhat like the removal of petrol from the crude material in that the latter also provides such substances as paraffin wax and vaseline, which would be quite useless in the cylinders of a motor car.
The efficiency of the motor car depends upon the complete combustion of the petrol, which reacts with the oxygen of the air. The products of this combustion are almost entirely carbon dioxide and water vapour, which escape through the valves as exhaust gas. Tissue respiration also requires oxygen, this being carried by the blood stream from the lungs. The main gaseous product in this case is also carbon dioxide, which is removed by the blood stream from the working parts. When the blood reaches the lungs it gives up its carbon dioxide, which is thereupon breathed out. In this respect the lungs function as both carburettor and exhaust.
The machine is so constructed by man that its parts work efficiently and cooperate with each other mechanically. In order to deal with unforeseen circumstances a certain amount of attention on the part of the driver is always necessary. The animal controls itself, becoming acquainted with changes in its surroundings and with its requirements by means of sense organs. The machine in the course of time becomes worn, but with living machines the repairs are carried out at the same time as wearing occurs, so that it is not apparent unless the balance of wear and repair is upset. This shows itself with the approach of old age, and up to the present no method is known whereby old age, and finally death, may be postponed indefinitely. The living machine is therefore ^// ’-controlled through the operation of its sense organs and nervous system.
A common misunderstanding regarding the process of tissue respiration is that the energy set free for a useful purpose during the process becomes lost. This is not so, for the energy while it is being utilized becomes transformed, and generally we can detect it as heat. Just as muscles when doing mechanical work become warm, so in like manner does electrical energy passing through the filament of an electric light bulb become transformed into heat until such a temperature is reached that some of it is transformed into light, or, as we commonly say, ’the wires become white hot. ’