Popular Science Monthly/Volume 20/April 1882/How Animals Breathe II
|HOW ANIMALS BREATHE.|
SPECIAL ORGANS OF THE FOOD-TRACT.—Another class of respiratory organs may now be distinguished, namely, those developed directly from the alimentary canal. Here belong the more highly specialized organs of the vertebrates.
Aquatic Organs of the Food-Tract.—The gills or branchiæ of fishes are analogous in position and structure to those of crabs, but are morphologically different, as they are not developed from the skin directly, but from the lining of the pharynx. A powerful heart impels the blood rapidly through the gills, while these are bathed by water-currents produced by the pumping action of the mouth; so that rapid and constant changes are effected in both the blood and the aërating water. The branchiæ are comb-like fringes of minute blood-vessels,
placed on bony arches, having a complex structure, and beautifully adapted to their purpose of exposing a great amount of blood in small space and in brief time.
It is impracticable to describe at length the various arrangements of gills in the different groups of fishes. Their origin from the food-tract is clearly shown in the lowest fish, the worm-like Amphioxus, which has gills formed from a large, barrel-shaped pharynx pierced with transverse slits through which the water is drawn by cilia (Fig. 2, page 645 of the September, 1881, "Monthly"). This creature differs from all other known vertebrates in the possession of cilia as the means of renewing the water.
The lamprey and hag, and the shark family, have the branchiæ in separate pouches developed from and connected with the pharynx; while most common fishes possess the familiar form.
In the embryonic state, the shark has external gills—long, filamentous appendages projecting through the gill-slits. But gills of this nature have a more remarkable and enduring character in the amphibia. In the toads, frogs, and most salamanders, these external gills give place early in life to internal fish-like gills, which are, in turn, replaced by lungs. But in certain species these external organs persist throughout the life of the animal; and the group is consequently known as the Perennibranchiata. The Proteus, of Austrian caves, has three pairs of scarlet fringes on each side of its neck. In the axolotl, of Mexico, the six gills are somewhat arborescent; while in the Menobranchus, found in the United States, they are plume-like. The Siren of the Carolina rice-swamps is another member of this group.
These plumose appendages upon the neck of the perennibranchiates would seem, judging simply by their appearance, to be as truly a part of the outer skin as any gills of the invertebrates. Indeed, such relation is much less apparent in the crabs and in the true snails. But, for reasons lying deeper than mere appearance, the gills of amphibians are classed with food-tract organs. However, this involves a nice distinction, since the food-tract and skin are fundamentally one.
Aërial Organs of the Food-Tract.—Dismissing now the water-respiratory organs of the food-canal, we pass to the consideration of its special air-breathing organs.
The habit among fishes of swallowing air has already been mentioned. It is probable that in many fishes this air, or a portion of it is simply passed through the gills, or perhaps is held in the oral cavity to aërate more highly the water that bathes the gills. However this may be, free air is so essential to many fishes that they die if prevented from obtaining it, especially in hot weather.
There is no organic reason why fishes could not breathe air if the gills could be kept moist and free. Indeed, there are a few fishes which even pass a great part of their lives out of water. "Such are the two genera, both belonging to the Gobiidœ, Periopthalmus and Boleopthalmus; these skip along close to the water-line on the sea-shore, where they hunt for mollusks and insects. In their branchial
cavity; like all fishes, they have true gills; but these, though not differing widely from those of other fishes living constantly in the water, are far from filling up the cavity, which is rather large; and this seems to contain not merely water but air as well." It will be seen that this respiration is analogous to that of the land-crabs. And no better illustration is needed to show the identity in principle of the two kinds of respiration, aquatic and aërial.
Another class of air-breathing fishes, of which the Anabas scandens, or climbing perch of India, is a famous example, have an upward extension of the branchial cavity containing complicated foldings of the skull-bones covered with mucous membrane, which remains moist either by secretion or by condensation of moisture from the air. The Anabas lives in ponds which are liable in times of severe drought to become only mud or even entirely dry. It then travels over scorched and dusty ground in quest of water, and has been kept alive without water for six days. Tropical fresh-water fishes are commonly "able to survive droughts, living in semi-fluid mud or lying in a torpid state
below the hard-baked crust at the bottom of a tank, from which every drop of water has disappeared."
The lung of vertebrates is an offset, "diverticulum," of the food-canal, and in some form is possessed by all classes of back-boned animals. In the fishes it is represented by the "swim-bladder," which is mechanical in function, serving to vary the specific gravity of the body. Yet in some species it has also a respiratory function. It is quite wanting in those fishes which, like the skate, grovel on the sea-bottom, and it is relatively large in the flying-fishes. In most adult
fishes the air-bladder is entirely closed, having no communication with any other organ; and the inclosed gas is obtained from the blood. This is largely nitrogen in fresh-water fishes, and oxygen in salt-water fishes; but the proportion of oxygen varies at different times, and is thought to act as a reserve against a time of need. This would be indirectly a respiratory function.
Certain fishes, the pike and eel for example, have the air-bladder communicating with the stomach, while the carp and many others have it in communication with the œsophagus. In the gar-pike, a cellular air-bladder communicates with the mouth by a trachea and glottis; and the mud-fishes have the same, not only resembling thus in structure the lungs of reptiles, but performing the same function. Thus we find in the single class of fishes a progression from a closed sac of mechanical function to a double lung like that of reptiles, and the point of communication rises from the stomach to the mouth.
Fig. 6.—Anabas scandem; head, with k the gill-cavity laid open, and l the contiguous cavity containing the foliated labyrinthine structure. b, Tadpole; c, young Polypterns from the Nile; d, embryo of the shark. All these have external gills, br. (After Semper.)
All amphibians possess lungs in the adult state, but with varying degree of usefulness. Those having permanent gills may use the lungs very little, as the Proteus and Menobranchus, some of them perhaps not at all; while others, as the Siren, for instance, use them mainly. Other amphibians without gills may also quite dispense with the lungs, using the skin instead. Even the frogs and toads can long survive the removal of their lungs. Many species of this class aid respiration by swallowing air. Frogs and toads force the air into their lungs by a swallowing action, made necessary by the absence of ribs; and they can be suffocated, and turtles also, by holding their mouth open.
In the amphibians, we have a class of animals making use of four distinct means of respiration; three of them—skin, gills, and lungs—in about equal degree. It might, therefore, be supposed that their respiration should be more active and the aëration of their blood more complete than in other vertebrates. Nevertheless, quite the reverse is the truth, for the reason that any function is better performed when localized or "specialized."
This latter fact is illustrated in the reptiles, which have a circulation as incomplete as the frog, with respiration more active, although
possessing only lungs. These are, of course, better developed in the reptile, where they are large sacs, having the interior surface increased by foldings, producing sacculi. Serpents have the left lung undeveloped, the right one forming a long, cylindrical sac capable of holding a large amount of air. By this means water-snakes are rendered buoyant, and fitted for long submergence. The last fact is also true of turtles.
Lizards and crocodiles have two lungs, usually somewhat divided, and extending through the whole trunk. By their inflation the chameleon can give itself a plump appearance.
Reptiles have only a slight motion of the ribs, and are remarkable as a class for feebleness of respiration, considering that their lungs are proportionately the largest among animals. In some species the skin is quite as active as the lungs, and the latter can be removed without causing immediate death.
The lungs of birds are less developed structurally than those of mammals, but are much larger. Connected with them in various parts of the body are air-sacs, especially in the abdomen and beneath the skin of the neck and wings. Except in water-birds, the hollow Fig. 9.—General view of the Air Reservoirs of the Duck, opened inferiorly; also their Relations with the principal Viscera of the Trunk. 1,1, anterior extremity of the cervical reservoirs; 2, thoracic reservoir; 3, anterior diaphragmatic reservoir; 4, posterior ditto; 5, abdominal reservoir.—a, membrane forming the anterior diaphragmatic reservoir; b, membrane forming the posterior ditto.— 6, section of the thoraco-abdominal diaphragm.—d, subpectoral prolongation of the thoracic reservoir; e, pericardium; f, f, liver; g, gizzard; h, intestines: m, heart: n, n, section of the great pectoral muscle above its insertion into the humerus; o, anterior clavicle; p, posterior clavicle of the right side cut and turned outward. (From M. Sappey’s work.) bones also contain air, and by their connection with the lungs respiration can be continued through an opening in the arm or thigh-bone, although the windpipe may be tied.
The respiratory system is most developed in birds of n powerful flight, and doubtless aids in rendering them buoyant. Perhaps the air-sacs beneath the wings assist in holding the latter outstretched; and it has been suggested that the sacs might serve as a cushion to protect those which suddenly dive into water after prey.
The blood-capillaries in the lungs of reptiles and amphibians are exposed to the air on one side only, while those of birds and mammals are arranged on a different and superior plan, being exposed on two opposite sides. Lungs of birds consist of an aggregation of distinct globules or "lunglets." As the lungs are attached to the dorsal side of the chest and the diaphragm is imperfect, expiration is effected by an active effort—by pulling the bone nearer the spine, and so diminishing the cavity.
While reptiles can live in air too impure for mammals, birds will die in an atmosphere which to mammals is quite harmless. Birds bear a relation to other vertebrates similar to that of insects among the invertebrates. They lead an insect-like existence; and their rapid respiration is effected not, as in mammals, by minute partitioning and subdivision of the lungs, but, as in insects, by extension and increase in capacity. And the air-cavities in the bones and tissues bring the air, as in insects, into effective contact with the capillaries of the system.
In mammals, respiration is quite restricted to the lungs, the skin performing in man only about one fiftieth part of the work. The lungs are less in proportionate bulk than those of reptiles and birds; but the lack of capacity is compensated by the minute subdivision of the cavity, giving immensely greater surface. The active carnivores possess the largest lungs.
The ultimate cells of the human lungs are from one two-hundredth to one seventieth of an inch in diameter, and in number are about six hundred million. Mammalian lungs are always partially filled with air, and only by great pressure can the air be sufficiently expelled from the lung-tissue to allow it to sink in water. This property has given the lungs the vulgar name of "lights."
Fig. 10. Bronchi and Lungs of Man. (Sappey.) 1, 1, summit of the lungs; 2, 2, base of the lungs; 3, trachea; 4, right bronchus; 5, division to the upper lobe of the lung; 6, division to the lower lobe; 7, left bronchus; 8, division to the upper lobe; 9, division to the lower lobe: 10, left branch of the pulmonary artery: 11, right branch; 12, left auricle of the heart; 13, left superior pulmonary vein; 14, left inferior pulmonary vein; 15, right superior pulmonary vein; 16, right inferior pulmonary vein; 17, inferior vena cava; 18, left ventricle of the heart; 19, right ventricle.
That portion of the contained air which in life can not be expelled from the lungs is called residual air. The amount of air moved in ordinary breathing employs but a small part of the breathing capacity, and is termed tidal air. Besides the residual air which can not be expelled, the lungs ordinarily contain a large quantity which can be exhaled by a forced expiration. This is known as reserve air. And, in addition to these three kinds, a large quantity can be inhaled by a forced inspiration, called complemental air.
In this can be seen a fine adaptation to the requirements of varying circumstances; for, if the breathing of mammals were normally at the full capacity, there would remain no provision for varying the rapidity of respiration according to temperature and exercise. Moreover, the residual air gives uniformity and constancy to the respiratory change of the blood, and prevents sudden variations in the kind, amount, and temperature of the air in the lung-cells, which would be injurious to the blood and to the delicate tissues. The change of the residual air is slowly effected by the physical process of diffusion between it and the reserve air. When this purifying change is not sufficiently rapid, we are impelled to take a "deep breath," and so wholly replace the reserve air.
The relative and the absolute amounts of each of the four kinds of respiratory air in the human lungs may be tabulated thus:
|Vital Capacity||Complemental air (can be inhaled by effort)||90-||110||cubic inches.|
|Tidal air (moved in ordinary breathing)||20-||30||""|
|Reserve air (can be exhaled by effort)||90-||110||""|
|Residual air (can never be exhaled)||100-||130||""|
|Total capacity of human lungs||300-||380||cubic inches.|
The breathing-pump of mammals may be compared to a conical box with movable sides and base. By contraction of the muscles attached to and connecting the ribs, the sides of the chest are moved upward and outward; while at the same time the diaphragm, forming the arched base of the chest, is depressed or flattened by its muscular contraction. Thus the greater muscular effort in ordinary breathing is used to enlarge the cavity of the thorax or chest, producing inspiration—not, as in birds, to diminish the cavity, producing expiration. In other words, the air is forced into mammalian lungs by atmospheric pressure when, through muscular effort, the chest enlarges; and the air is expelled simply by the elastic reaction of the lungs and chest.
The lungs are freely suspended by the windpipe, and are distended by the atmospheric pressure in opposition to their elasticity. Consequently, an opening in the walls of the thorax is liable to produce suffocation, by giving the air a more direct and easy route to the vacuum of the chest than through the trachea and lungs.
A delicate membrane called the pleura closely invests the lungs, and is then reflected to line the cavity of the thorax. By the secretion of a serous fluid, it prevents friction, which would otherwise result from the constant movement. A rude apparatus to illustrate the mechanism of breathing in mammals is easily made by suspending the lungs of some small animal in a glass bell-jar closed below by an elastic membrane—a sheet of rubber, for instance—the only access for air to the interior being through the windpipe. Then the forced enlargement of the cavity, by pulling down the membrane, causes the inflation of the lungs. This apparatus is deficient in mobility of the walls.
The rapidity of the respiratory movements in man is about one inspiration to four heart-beats, or fifteen to twenty-five per minute; greatly varying, however, according to age, sex, and circumstances. In animals with high temperature, breathing is much faster, becoming almost a tremor in birds. In the w T hale, on the contrary, breathing is suspended while the animal is under water; it being provided with reservoirs of pure blood. When the latter is exhausted, the creature comes to the surface and puffs and "blows" to obtain air and refill the reservoirs.
The difference in color of the blood of vertebrates is chiefly due to the varying amount of oxygen in chemical combination with the hæmoglobin of the red corpuscles—the brightness of color being proportionate to the oxygen. An essential part of the hæmoglobin is iron; and it has been supposed that the change in color is due to a chemical change from a ferrous to a ferric salt. But this simple and plausible explanation is now denied by eminent physiologists, who, however, admit that the iron has some essential but unknown influence. A minor cause of the darker color of the blood is the swelling of the corpuscles by absorption of carbonic acid.
The corpuscles are the oxygen-carriers, seizing the oxygen in the lungs and conveying it to the tissues, where it unites with the carbon
and hydrogen. The corpuscles also convey carbonic acid to the lungs, but they divide this labor with the serum.
In large quantity, carbonic acid acts as a narcotic poison; for, on account of its superior attraction for the hæmoglobin, it replaces and excludes the oxygen. Other gases, as nitric oxide, have the same effect. Nitrogen, on the other hand, is entirely negative in its effect on the blood, and consequently serves to dilute the oxygen of the atmosphere, without injury to animal life.
More oxygen is inhaled than is exhaled as a component of carbonic acid. The extra amount doubtless unites with hydrogen to form a portion of the exhaled water, and to produce sulphuric and phosphoric acids.
The human lungs exhale, in twenty-four hours, about two pounds of carbonic acid. This is the product of the burning of nine ounces of carbon. As giving some idea of the forces within the body, it is interesting to know that the combustion of nine ounces of carbon liberates over six million foot-pounds of energy. This is equivalent to more than one eighth of a horse-power acting continuously for the twenty-four hours; or it equals one hundred and eighty-two horse-powers working for one minute.
But the combustion of carbon does not include the total oxidation within the body; for, in less degree, hydrogen, sulphur, phosphorus, and iron, are also burned.
A large part of the energy thus produced is utilized in the unceasing labor of the circulation and the respiration. In a year the number of respirations is, in most persons, over nine million; and one hundred and twenty-five thousand cubic feet of air carried through the lungs purifies at least five thousand tons of blood. Yet, so perfect is the apparatus, that we are almost unconscious of its action, unless warned by disease, or the delicate lining of the air-tubes is irritated by some foreign matter.
- Karl Semper's "Animal Life," p. 189.
- Gunther's "Introduction to the Study of Fishes," p. 24.