THE NATURALIST.

Otago Witness, Issue 2437, 28 November 1900, Page 62

THE NATURALIST.

HOW AKIMALS BREATHE.

■A NATURAL SCIENCE STUDY. By E. Stbnhotjse, A.E.C.S., B.Sc. It is difficult now to realise the dismay witb which the views of Darwin and his disciples were at first received. Before the appearance of "The Origin of Species," most men supposed that human beings stood apart from the-rest of animate nature, and they apparently derived' considerable satisfaction from the thought. Nowadays naturalists find pleasure in the belief that all life is linked together by an actual "blood relationship," and that every organ and function, 'however complete it may seem in itself, is- but one link in a great life-chain, one term in a great life-series. In the present article I propose to consider respiration — one of the most important functions" of all living things — from this point of view, and to show, briefly, that the_ process of (breathing is essentially the same whether its outward evidence be the gulping of a frog, the tailwagging of a dragon fly larva, or the tremulous palpita- ] tions of a bee. The necessity for breathing must first ', be explained. The energy which enables a muscle to contract is derived from the oxidation — the slow burning — of part of its substance ; j\ist as the energy which enables a steam engine to move is derived from the burning of fuel in the boiler fires. The fires soon go out, and the engine stops, unless fresh fuel is added from time to time and a plentiful supply of air is available. So the muscle loses its power of contracting unless the waste matters resulting from previous contractions are cleared away and replaced by fresh food and fresh oxygen. Wherever vital action is taking place, whethei in a contracting muscle, a secreting gland, or a thinking brain, there is continual consumption of oxygen, and continual production of waste material, chiefly carbon dioxide. In the higher animals the renewal of oxygen and the removal of carbon dioxide are performed by a circulating fluid, the blood. Blood vessels are to Jhe body what rivers and canals are to a country ; they act as highways for the transportation of material. We may, perhaps, carry the analogy a little farther, and find in the "red corpuscles" of tlie blood the rough equivalents of the canalbarges, for they :arry along with them tiny loads of oxygen. As the blood creeps along its narrow channels in an active tissue, the red corpuscles relinquish their oxygen, and the fluid portion of the blood takes up carbon dioxide. The blood continues its course, and sooner or later arrives at a place where it can obtain a fresh supply of oxygen, and get rid of its extra carbon dioxide. In ourselves this change takes j>lace_a§ bjood as jjassuy* through the

■small vessels of the "lungs. There the blood is separated from the air by a membrane so delicate that gases can readily pass through it ; and hence, on leaving the lungs, the blood has got rid of the waste carbon dioxide, and its red corpuscles are laden with fresh oxygen. This exchange of useless carbon dioxide for oxygen constitutes respiration, and that part of an animal in which the change takes place is, consequently, called a respiratory organ. The habits of animals, and the conditions under which they live," are so varied that several different devices for obtaining oxygen are in vogue. Air-breathers generally possess lungs. These are sacs open to the air. ' They may be quite simple, or they maj <oe branched in a, more or less complicated manner: an arrangement which greatly increases the extent of their exposed surface. In all cases their walls are richly supplied with blood-vessels. Diffusion takes place so freely that in active animals the air inside the lungs soon be'eomes vitiated, unless there is some means of changing it. Under ordinary conditions a man "breathes," or changes the air in his ■brags, froof 13 to 15 times a minute. He does this quite mechanically, and without thinking about it. Every four seconds or so a set of muscles contracts and pulls his ribs upwards and outwards ; another muscular contraction pulls down the floor of his ches_t. As a consequence, the cavity is much enlarged. - .The- ''lungs -follow the movements of the walls of fhe ' chest, and some 30 cubic inches of air are .sucked in. Immediately th¥ ribs fall back to their former position, the chest-floor rises, and air is driven out. Then, after a short pause, the process is repeated. It should be mentioned that only about 30 cubic inches of air are changed at each respiration, although the capacity of the lungs averages about 230 cubic inches. All mammals breathe in much the same way. Birds, however, have a somewhat different metnod. The main air-tubes which enter the lungs do not simply break up into fine divisions and there end, but pass, on and form large air-sacs in different parts of the body, which actually communicate witn spaces inside the bones. When therefore, the air-sacs alternately dilate and contract, air is pumped in and out of the j lungs so completely that in this region it is almost wholly renewed at .each respira- ] tion. This, coupled with the fact that the air is distributed to all parts of the body, ensures -very perfect aeration of the J blood of birds, and accounts for their high temperature. The air of a frog's lungs' is renewed ,in a very interesting manner. The mouth is kept 'dosed ; -its floor is depressed, and air is drawn in through the nostrils. The nostrils are then shut, and, when the floor of the mouth is again raised, the air is forced down into the lungs. The process may be compared to the action of a forcepump, while a man's breathing resembles that of a suction pump. The blood of a frog is, however, probably aerated quite as much, in the skin as in the lungs. In their early stages frogs and other amphibia i lead aquatic lives, in which lungs would be useless. The medium rwhich surrounds them is water, and their only source of oxygen is the air dissolved in the water. This dissolved air is made use of by means of gills which grow on the sides of the neck. The gills are very abundantly supplied with blood-vessels, and their delicate walls readily allow oxygen to pass from the water to the blood, and carbon dioxide from the blood to the water. This method of •breathing is the normal one among the fishes, and it is reasonable to suppose that the close correspondence, in structure and habits, of the young amphibians to true fishes indicates that amphibians are descended from fishes. A link between the two classes (fishes and amphibians) is found in the very curious mud fish of Queensland. This creature possesses not only gills, but also a true lung, and it uses both for purposes of respiration. The climbing perch is so accustomed to a life on land that it may be drowned by prolonged immersion in water ; and another "fish, called Periophthalnius, is in the habit of perching on a rock, and breathing by means of its tail-fin, which dips in the water. These instances show that breathing may be carried on in several different ways. Further examples are abundant in the lower animals. Most of the mollusca, for example, are provided with delicate outgrowths, genei'ally - known as ■ gills, which receive a rich Wood-supply. These vary greatly in appearance. They may resemble plumes and brushes, or — as in the case of the mussel — form a fine basket-work, through the meshes of which the water is driven by the lashing of thread-like cilia. The air-breathing snails and their relatives, have a chamber which corresponds precisely in function to a lung. The life of insects is in general so active that very complete respiration is necessary. This is effected by mSans of systems of airtubes which ramify throughout the whole 'body. The ultimate divisions of these tubes are so fine that oxygen is supplied directly co the interior of every organ. It will be noticed that in this instance air is 'arried to the blood, whilst in all the cases mentioned above the blood is carried to the air. The air of the tubes is renewed by alternate contractions and dilatations of the body, easily noticed in a bee or wasp. Many of the insects pass through an aquatic larval stage, in which air-breath-ing would be impossible. The difficulty is overcome in some sases by the development^ plate-Uke outgrowths, called tracneal gills. These take up oxygen from the water. It is interesting to notice that birds and insects, which alone are normally adapted for flight, have the most perfect breathing apparatus. The scorpions and the spiders have respiiatory organs of a type quite different ' from any of those already described. On the sides of the scorpion's body are four pairs of slits, each of which leads into a chamber. The lining of the chamber is raised into a number of thin plates, much resembling the^ leaves of a book. The leaves are well provided with blood, which j is oxjtgemlej. from the^ a« cir^uMjnj^be-

Ween them. fhecEanibers are known to naturalists as book-lungs. Crustaceans generally breathe by gills. These vary in form, but are commonly plume-like, and may be enclosed in a gillchamber. The %ills ( are often attached to, or are outgrowths* of, the legs, and the movements -of the latter greatly assist respiration. If a living crayfish be watched, it will be seen that in front of the gillchambei a bent plate is continuously per-, forming a scooping motion, which keeps a regular current of water flowing over the gills. The lowest a,nimals, and some — the earthworm, for example— that are of relatively high organisation, breathe through the gene* ral surface of the body, and have no spe* cial organs of respiration. ! Whatever means be adopted, however,the essential features of the process are the same throughout. Reduced to its simplest expression, respiration is an exchange, for fresh oxygen, of the carbon dioxide which, lias been produced by the activity of a living organism ; ' and the respiratory organ is the thin membrane through which the exchanging gases pass.