The mode of motion requiring the greatest expenditure of energy is that of flying. It is not surprising, therefore, to find that of all those respiratory systems where the actual respiratory organs are the lungs, that possessed by birds is the most highly developed. The mechanism not only consists of those parts found in mammals, except for the diaphragm, but there are in addition large spaces opening out by narrow tubes from the lungs. These occupy the available space in the thorax, abdomen, and one even lies inside each humerus bone. The lungs are the only organs where gaseous exchange occurs. The ribs are raised by muscles at the same time as the abdominal muscles contract. This double action causes the air sacs to be compressed from behind so that air rushes forwards through the lungs, whereupon gaseous exchange occurs. The collapse of the ribs and 8A the relaxing of the abdominal muscles causes the air sacs to fill again. During flight the wing muscles prevent the rib muscles from being used, but the filling and emptying of the air sacs is carried out at intervals by the abdominal mechanism.
The respiratory organs in a snake are the lungs, and the mechanism for filling and emptying them is exactly the same as that of a mammal, except that there is no diaphragm. The left lung is very small and the right one greatly elongated. Both are spongy, the hinder part being quite smooth, but possessing a central cavity throughout. This is a type of lung intermediate in structure between that of the frog and die mammal.
The adult frog breathes by three methods. Its skin is plentifully supplied with capillaries, and so long as the skin is kept moist the exchange of gases occurs quite readily. Experiments have shown that for normal activities 50 per cent, of its breathing occurs through the skin. It is therefore understandable why captive frogs are much more lively immediately after shedding their outer skin.
In the lining of the mouth are many capillaries. Air entering through the nostrils into the mouth gives up its oxygen to these capillaries which liberate carbon dioxide at the same time. The third breathing mechanism involves the lungs. These are not used very much unless the animal is very active, leaping or swimming. The parts concerned with mouth-breathing consist of the nares, which connect directiy with the exterior, muscles which run across the floor of the mouth and muscles running from the tip of the lower jaw to the breast region. These last muscles, which are connected with a little plate in the floor of the mouth known as the hyoid, are pulled down when they contract, thereby lowering the floor of the mouth, thus increasing its volume and causing air to enter through the nares. When the transverse muscles contract the mouth cavity is made smaller, the pressure inside is thereby increased, and so air is forced out through the nares. The muscles involved work alternately, the transverse muscles contracting while the hyoid ones relax, and vice versa.
The parts involved in lung-breathing include those required for mouth-breathing, with the addition of valves and the lungs. A valve is present in each naris, and another is a slit running from front to back in the thin flexible membrane which forms the floor of the pharynx. The glottis leads to a chamber, which opens out on each side to a lung, a simple bag with slightly infolded walls and richly supplied with blood vessels. Except during lung-breathing, the glottis is shut.
When empty of air each lung is a small grey sac, pointed at its posterior end, hut on inflating it with air it considerably increases in size. If punctured it immediately shrinks again owing to its elasticity. Blood is carried to the lungs from the heart by the pulmonary arteries and returns again by the pulmonary veins. is raised into a pair of folds, the vocal cords. When these are lying parallel air passing through the larynx causes them to vibrate and so produce the characteristic croaking of the frog.
To pass air into the lungs the hyoid muscles contract, the floor of the mouth becomes lowered, the pressure inside is decreased, and air passes in through the nares. The nares are now shut. The hyoid muscles relax as the transverse muscles contract, compressing the air inside the mouth. This pressure when sufficient causes the membrane in the pharynx to cave in downwards, opening the glottis, and air passes into the lungs.
Thus the air is really forced into the lungs by the pumping action of the mouth, the air in the lungs being now under pressure greater than atmospheric. Gaseous exchange now occurs. Then the nares are opened, allowing the air in the mouth to escape. The air in the lungs causes the membrane to bulge upwards, opening the glottis, and the air rushes out aided by the natural elasticity of the lungs.
For the purpose of showing breathing movements by living fish, gold-fish or cod are suitable. These animals when not rapidly swimming are continually moving the large plate, or operculum, which lies behind and below each eye. The pectoral fins, when the fish is still, also carry out regular waving movements, while the mouth opens and shuts. All these movements result in a current of water passing into the mouth. For examining the parts of the breathing mechanism the herring or cod is best. Byaway the operculum with sharp scissors the gills are exposed. The use of a blunt instrument shows that there is a passage leading from the mouth through the gill slits to the exterior at the operculum. The gill slits are openings in the side walls of the pharynx, which is supported there by small rods of bone forming gill arches. The outer border of each gill arch bears a double row of gill filaments projecting into the cavity beneath the operculum. Each filament is a horizontal, delicate pink thread-like structure and its surface is corrugated into numerous platelets which expose an enormous area to the surrounding water and which are richly supplied with blood by a network of capillaries. The inner border of each gill arch bears numerous small spines, the gill rakers, which serve to protect the delicate gills from food particles which might pass out through the gill slits. In the herring the gill rakers are long and densely packed bristles forming a most effective sieve. They are so sharp in the cod that care must be taken when examining its gills. To breathe, the fish opens its mouth and the floor is lowered, each operculum being held against the side by the pressure of the water outside. Water passes in through the mouth. The opercula are now moved away from the sides as the mouth shuts and its floor is raised, the water being forced to pass between the gills, where gaseous exchange occurs, and flows out of the animal behind each operculum. The exchange of gases is carried out by the oxygen diffusing into the gills as the carbon dioxide diffuses out similarly into the surrounding water.
There is a large cavity inside insects in which the various organs of the body lie. It is filled with blood, which is colourless, so that the organs are bathed in it, rendering unnecessary the capillaries found in animals which possess red blood. Minute tubes ramify throughout the parts lying in this fluid and join together forming larger tubes near the surface of the body, opening to the exterior by apertures called spiracles, which are protected by valves and by bristles to which solid particles will stick. In the cockroach there are three pairs of spiracles in the thorax and seven pairs in the abdomen. The air is caused to pass into and out of the trachea? by rhythmic movements of the body. The dorsal and ventral surfaces are drawn closer together and then separate, while in some insects the abdomen becomes somewhat telescoped and then extended. This telescoping movement can be readily observed in a wasp, occurring about 100 times per minute. Some authorities claim that there is a regular circulation through the body since air seems to enter by spiracles in the thorax and to leave by those in the abdomen. It is not yet known how air is passed along the tracheoles, although a certain amount can move by gaseous diffusion along them. In fact it is not really fully understood how the rhythmic movements of the body cause the air to move into and out of the tracheae, which have somewhat rigid walls. The tracheoles pass throughout the tissues to form a very complex network which may contain fluid, but it is important to note that the oxygen can reach nearly all tissues as a gas. In the trachea? there are thin-walled expansions, called air sacs, which may supply air for use during flight, since tissue respiration is then speeded up tremendously and more oxygen will therefore be required.
There are no special respiratory organs in the earthworm, but it obtains the necessary oxygen through the skin, which is very thin and flexible and covered by a very thin and glistening cuticle. The skin is plentifully supplied with blood vessels, and is kept moist by various tiny glands in its surface, as well as by the fluid which is poured out through the dorsal pore in each segment. Since the animal does not carry out violent activities like running, leaping, swimming or flying, it is able to get sufficient oxygen for its needs in this way.
Hydra, Paramecium and Amoeba
These lowly forms of animal life possess no structures which are specialized for obtaining oxygen and getting rid of carbon dioxide. This may be explained by the fact that although for their size they are quite active, the proportion of their surface area to their volume is extremely large. This large surface area enables them to obtain ample supplies of oxygen from the water in which they live. The outer layer of the protoplasm readily allows the diffusion of gases through it.
Experiment 18—To illustrate the above Statement
In the case of Hydra, plasticine is moulded to form a 1-in. cube. It therefore has a volume of 1 cub. in. and an area of 6 sq. in. Pieces of it are then put aside, sufficient to make the tentacles and foot of a model Hydra. The remainder is flattened into a thin rectangular sheet, and this sheet is moulded to form the cylinder which will represent the body of the animal. The tentacles and foot are fastened on. By measuring the length and diameter of the cylinder its area may be calculated. This result is doubled, since the animal obtains oxygen from inside as well as outside itself. Even ignoring the area of the tentacles, this increase is surprising.
The reason why the relation between surface area and volume is so important where gaseous exchange is concerned is that the fundamental feature of this exchange is the rates of diffusion of oxygen and of carbon dioxide. This diffusion is a purely physical process, not a biological one, being governed only by the concentration of the gas and by the temperature. Quite obviously living cells can obtain oxygen much more readily if they are in thin sheets rather than bulky masses. The fundamental purpose of a blood stream is to convey materials from places where they can diffuse through walls of capillaries into the blood vessels to places where they can diffuse out. Tissues which are situated some distance from respiratory surfaces can obtain their supplies of oxygen and get rid of carbon dioxide without the gas having to diffuse all the way from the respiratory surfaces. In large organisms, therefore, the possession of a transport system of some kind is essential if only to supply the necessary oxygen and remove carbon dioxide.