Chapter VIII: Of Respiration (1)

Respiration in the plant; in the animal—Aquatic and aërial respiration—Apparatus of each traced through the lower to the higher classes of animals—Apparatus in man—Trachea, Bronchi, Air Vesicles—Pulmonary artery—Lung—Respiratory motions: inspiration; expiration—How in the former air and blood flow to the lung; how in the latter air and blood flow from the lung—Relation between respiration and circulation—Quantity of air and blood employed in each respiratory action—Calculations founded on these estimates—Changes produced by animal respiration on the air: changes produced by vegetable respiration on the air—Changes produced by respiration on the blood—Respiratory function of the liver—Uses of respiration.

313. No organized being can live without food and no food can nourish without air. In all creatures the necessity for air is more urgent than that for food, for some can live days, and even weeks, without a fresh supply of food, but none without a constant renewal of the air.

314. The food having undergone the requisite preparation in the apparatus provided for its assimilation, is brought into contact with the air, from which it abstracts certain principles, and to which it gives others in return. By this interchange of principles the composition of the food is changed: it acquires the qualities necessary for its combination with the living body. The process by which the air is brought into contact with the food, and by which the food receives from the air the qualities which fit it for becoming a constituent part of the living body, constitutes the function of respiration.

315. In the plant, the air and the food meet in contact and re-act on each other in the leaf. The crude food of the plant having in its ascent from the root through the stalk, received successive additions of organic substances, by which its nature is assimilated to the chemical condition of the proper nutritive fluid of the plant (320 and 325), undergoes in the leaf a double process; that of Digestion and that of Respiration. The upper surface of the leaf is a digestive apparatus, analogous to the stomach of the animal; the under surface of the leaf is a respiratory apparatus, analogous to the lung of the animal. For the performance of this double function, incessantly carried on by the leaf, its organization is admirably adapted.

View of the net-work which forms the solid structure of the leaf, and which consists partly of woody fibres, and partly of spiral vessels. 1. Vessels of the upper surface; 2. vessels of the under surface; 3. distribution of the vessels through the substance of the leaf; 4. interspaces between the vessels occupied by parenchyma or cellular tissue.]

316. The solid skeleton of the leaf consists of a net-work composed partly of woody fibres and partly of spiral vessels which proceed from the stem, and which are called veins (fig. CXXII. 1, 3). In the interstices between the veins is disposed a quantity of cellular tissue, termed the parenchyma of the leaf (fig. CXXII. 4): the whole is enveloped in a membrane, called the cuticle (fig. CXXIII. 1), which is furnished with apertures denominated stomata, or stomates (fig. CXXIV.).

Vertical section of the leaf as it appears when seen highly magnified under the microscope. 1. Cells of the cuticle filled with air; 2. double series of cylindrical cells occupying the upper surface of the leaf filled with organic particles; 3. irregular cells forming a reticulated texture occupying the under surface of the leaf; 4. interspaces between the cells, termed the intercellular passages or air chambers.]

317. The cuticle consists of a layer of minute cellules, colourless, transparent, without vessels, without organic particles of any kind, and probably filled with air (fig. CXXIII. 1). These cellules open externally, at certain portions of the cuticle, by apertures or passages which constitute the stomates (fig. CXXIV.), and which present the appearance of areolæ with a slit in the centre (fig. CXXIV.). They form a kind of oval sphincters, which are capable of opening or shutting, according to circumstances, and they are disposed on both surfaces of the leaf, but most abundantly on the under surface, excepting in leaves which float on water, in which they are always on the upper surface only.

View of the stomata of a leaf, some of them represented as open and others as closed.]

318. The cellular tissue or parenchyma, immediately beneath the cuticle, when examined in thin slices, and viewed under a microscope with a high magnifying power, presents a regular structure disposed in perfect order. It consists, on the upper surface, of a layer, and sometimes of two and even three layers, of vesicles of an oblong or cylindrical form, placed perpendicularly to the surface of the leaf, set close to each other (fig. CXXIII. 2), and filled with organic particles constituting the green matter which determines the colour of the leaf. On the under surface, on the contrary, the vesicles, which are larger than the cylindrical, are of an irregular figure, and are placed in an horizontal direction, at such distances as to leave wide intervals between each other (fig. CXXIII. 3); yet uniting and anastomosing together, and thus forming a reticulated tissue, presenting the appearance of a net with large meshes (fig. CXXIII. 3).

319. A leaf, then, consists of a double congeries of vesicles containing organic particles, penetrated by woody fibre and air vessels (which is probably the true nature of the spiral vessels), the whole being enclosed within a hollow stratum of air-cells.

320. The crude sap, composed principally of water, holding in solution carbonic acid, acetic acid, sugar, and a matter analogous to gum, is transmitted through the leaf-stalk to the cylindrical vesicles of the upper surface of the leaf (fig. CXXIII. 2). These vesicles exhale a large proportion of the water; the evaporation of which is so powerfully assisted by the action of the sun’s rays, that it would probably become excessive, were it not for the perpendicular direction of the cylindrical vesicles (fig. CXXIII. 2); but in consequence of their being disposed perpendicularly to the surface of the leaf, their ends only are presented towards the heavens (fig. CXXIII. 2), and thus the main part of their surface is protected from the direct influence of the solar rays. The primary effect of the evaporation carried on in the cylindrical vesicles, is the condensation of the organic matters contained in the sap.

321. At the same time that the cylindrical vesicles pour the superfluous water of the sap into the surrounding atmosphere, they abstract from the atmosphere in return carbonic acid, which, together with that already contained in the sap, is decomposed. The oxygen is evolved; the carbon is retained. The physical agent by which this chemical change, which constitutes the digestive process of the plant, is effected, is the solar ray; hence the vesicles which contain the fluid to be decomposed, are placed on the upper surface of the leaf, where their contents are fully exposed to the action of the sun; and hence also this process takes place only during the day, and most powerfully under the direct solar ray: but although the direct influence of the sun be highly conducive to the process, yet it is not indispensable to it; for it goes on in daylight although there be no sunshine. Light, then, would appear to be the physical agent which effects on the crude food of the plant a change analogous to that produced on the crude food of the animal by the juices of the stomach.

322. After the sap has been elaborated in the cylindrical vesicles, by the exhalation of its watery particles, by the condensation of its organic matter, by the retention of carbon and the evolution of oxygen, it is transmitted to the reticulated vesicles of the under surface of the leaf (fig. CXXIII. 3), These vesicles, large, loose, and expanded, as they have an opposite function to perform, are arranged in a mode the very reverse of the cylindrical: in such a manner as to present the greatest possible extent of surface to the surrounding air (fig. CXXIII. 3): at the same time the broad interspaces between them (fig. CXXIII. 4) are so many cavernous air-chambers into which the air is admitted through the stomates (fig. CXXIV.). The cylindrical vesicles, exposed to the direct rays of the sun, are protected by the closeness with which they are packed; and by the small extent of surface they present to the heavens: the reticulated vesicles, whose function requires that they should have the freest possible exposure to the surrounding air, are protected from the solar ray, first by their position on the under surface of the leaf; and, secondly, by the dense and thick barrier formed by the stratum of cylindrical vesicles (fig. CXXIII. 2).

323. In the cylindrical vesicles carbonic acid is decomposed; in the reticulated vesicles, on the contrary, carbonic acid is re-formed. The oxygen required for this generation of carbonic acid is abstracted partly from the surrounding air; the carbon is derived partly, perhaps, from the air, but chiefly from the digested sap, and the carbonic acid, formed by the union of these elements, is evolved into the surrounding atmosphere.

324. This operation, which is strictly analogous to that of respiration in the animal, in which carbonic acid is always generated and expired, is carried on chiefly in the night. In this manner, under the influence of the solar light, the leaf decomposes carbonic acid; retains the carbon and returns the greater part of the oxygen to the air in a gaseous form. At night, in the absence of the solar ray, the leaf absorbs oxygen, combines this oxygen with the materials of the sap to produce carbonic acid, which, as soon as formed, is evolved into the surrounding air. The carbonic acid gas exhaled during the night is re-absorbed during the day and oxygen is evolved; and this alternate action goes on without ceasing; whence the plant deteriorates the air by night, by the abstraction of its oxygen and the exhalation of carbonic acid; and purifies it by day by the evolution of oxygen and the abstraction of carbonic acid.

325. The result of these chemical actions is the conversion of the crude sap into the proper nutritive juice of the plant. When it reaches the cylindrical vesicles, the sap is colourless, not coagulable, without globules, composed chiefly of water holding in solution carbonic and acetic acids, sugar, gum, and several salts; when it leaves the reticulated vesicles it is a greenish fluid, partly coagulable and abounding with organic particles under the form of globules. Its chemical composition is now wholly changed; it consists of resinous matter, starch, gluten, and vegetable albumen. It is now thoroughly elaborated nutritive fluid; the proper food of the plant (cambium); rich in all the principles which are fitted to form vegetable secretions: it is to the plant what arterial blood is to the animal, and like the vital fluid formed in the lung, the cambium elaborated in the leaf, is transmitted to the different parts and organs of the plant to serve for their nutrition and development.

326. The formation of this nutritive fluid by the plant is a vital process, as necessary to the continuance of its existence, as the process of sanguification is necessary to the maintenance of the life of the animal. If the plant be deprived of its leaves, if the cold destroy, or the insect devour them, the nutrition of the plant is arrested; the development of the flowers, the maturation of the fruit, the fecundation of the seeds, all are stopped at once, and the plant itself perishes.

327. The proper nutritive juice of the plant, completed by the process of respiration, is formed by the elaboration of organic combinations of a higher nature than those afforded by the sap. Acid, sugar, gum (325) are converted into the higher organic compounds, resin, gluten, starch, albumen, probably by chemical processes, the result of which is the inversion of the relative proportions of oxygen and carbon. In the organic matters contained in the sap, the proportion of oxygen, compared with that of carbon, is in excess; on the contrary, in the higher compounds contained in the cambium, the carbon preponderates: by the inversion of the relative proportions of these two elements, the organic compounds of a lower nature, appear to be changed into those of a higher; to be brought into a chemical condition nearer to that of the proper substance of the plant; a condition in which they receive the last degree of elaboration preparatory to their conversion into that substance.

328. In the process of respiration in the animal, as in the plant, parts of the digested aliment mix with the air; parts of the air mix with the digested aliment; and by this interchange of principles, the chemical composition of the aliment acquires the closest affinity to that of the animal body; is rendered fit to combine with it; fit to become a constituent part of it.

329. The extent and complexity of the respiratory apparatus in the animal, is in the direct ratio of the elevation of its structure and the activity of its function, to which the quantity of air consumed by it is always strictly proportionate.

330. The process of respiration in the animal is effected by two media, air and water; but the only real agent is the air; for the water contributes to the function only by the air contained in it. Respiration by water is termed aquatic, that by the atmosphere, atmospheric or aërial respiration.

331. The quantity of air contained in water being small, aquatic is proportionally less energetic than aërial respiration; and, accordingly, the creatures placed at the bottom of the animal scale, having the simplest structure and the narrowest range of function, are all aquatic.

332. Whatever the medium breathed, respiration in the animal is energetic in proportion to the extent of the respiratory surface exposed to the surrounding element. As the water-breathing animals successively rise in organization, their respiratory surface becomes more and more extended, and a proportionally larger quantity of water is made to flow over it. It is the same in aërial respiration: the higher the animal, the greater the extent of its respiratory surface; and the larger the bulk of air that acts upon it.

333. Whatever the medium breathed, respiration is effected by the contact of fresh strata of the surrounding element with the respiratory surface. The mode in which this constant renewal of the strata is effected, is either by the motion of the body to and fro in the element; or by the creation of currents in it, which flow to the respiratory surface. A main part of the apparatus of respiration consists of the expedients necessary to accomplish these two objects; and that apparatus is simple, or complex, chiefly according to the extent of the mechanism requisite to effect them.

334. Whatever the medium breathed, the organic tissue which constitutes the essential part of the immediate organ of respiration is the skin. The primary tissue of which the skin is composed is the cellular (23 et seq.), which, organized into mucous membrane (33 et seq.), forms the essential constituent of the skin (34). In all animals the skin covers both the external and the internal surfaces of the body (34). When forming the external envelop, this organ commonly retains the name of skin; when forming the internal lining, it is generally called mucous membrane; and in all animals, from the monad to man, either in the form of an external envelop, or an internal lining, or by both in conjunction, or by some localization and modification of both, the skin constitutes the immediate organ of respiration. In different classes of animals it is variously arranged, assumes various forms, and is placed in various situations, according to the medium breathed, and the facility of bringing its entire surface into contact with the surrounding element; but in all, the organ and its office are the same: it is the modification only—that modification being invariably and strictly adaptation, which constitutes the whole diversity of the immediate organ of respiration.

335. At the commencement of the animal scale, in the countless tribes of the polygastrica (vol. i. p. 34, et seq.), respiration is effected through the delicate membrane which envelops the soft substance of which their body is composed. The air contained in the water in which they live, penetrating the porous external envelop, permeates every part of their body; aërates their nutritive juices; and converts them immediately into the very substance of their body. They are not yet covered with solid shells, nor with dense impervious scales, nor with any hard material which would exclude the general respiratory influence of water, or render necessary any special expedient to bring their respiratory surface into contact with the element.

336. But in some tribes even of these simple creatures there is visible by the microscope an afflux of their nutritive juices to the delicate pellicle that envelops them, in the form of a vascular net-work, in which there appears to be a motion of fluids, probably the nutritive juices flowing in the only position of the body in which they could come into direct contact with the surrounding element. In some more highly advanced tribes, as in wheel animalcules, there is an obvious circulating system in vessels near the surface of the skin. In other tribes, the internal surface constituting the alimentary canal, is of great extent and width, and forms numerous cavities which are often distended with water. In this manner a portion of the internal, as well as the external surface is made contributary to the function of respiration, and this extended respiration is conducive to their great and continued activity, to their rapid development, and to the extraordinary fertility of their races.

1. The mouth; 2. the stomach; 3. large canals going from the stomach; 4. smaller canals which form; 5. a plexus of vessels at the margin of the disc serving for respiration; 6. margin of the disc.]

337. In creatures somewhat higher in the scale, a portion of the external surface is reflected inwards in the form of a sac, with an external opening (fig. CXXV. 1). In some medusæ there are numerous sacs of this kind, which pass inwards until they are separated only by thin septa from the cavities of the stomach. The water permeating and filling these sacs comes into contact with an interior portion of the body, not to be reached through the external surface. At the margin of the disk (fig. CXXV. 6) there is spread out a delicate net-work of vessels (fig. CXXV. 5); these vessels communicate with small canals (fig. CXXV. 4) which open into larger canals (fig. CXXV. 3) that proceed directly from the stomach (fig. CXXV. 2). As the aliment is prepared by the stomach, it is transmitted thence by these communicating canals to the exterior net-work of vessels where it is aërated.

338. As organization advances, as the component tissues of the body become more dense, and are moulded into more complex structures, when, moreover, these structures are placed deep in the interior of the body, far from the external envelop, and proportionally distant from the surrounding element, the respiratory apparatus necessarily increases in complexity. The first complication consists in the formation of minute, delicate, transparent tubes (fig. CXXVI. 5), which communicate with the external surface by a special organ (fig. CXXVI. 4) that conveys water into the interior of the body (fig. CXXVI. 5). By means of these ramifying water-tubes, upon the delicate walls of which the blood-vessels are spread out in minute and beautiful capillaries, the water is brought into immediate contact with the vascular system.

1. Mouth; 2. salivary sacs; 3. intestine; 4. cloaca; 5. ramified tubes, conveying water for respiration into the interior of the body.]

339. Next, in the ascending scale, the external envelop of the body is extended into a distinct additional or supplemental organ, by which the function of the skin is assisted. This additional organ is called branchia or gill. The simplest form of branchia consists of folds or duplicatures of skin, forming ramified tufts (fig. CXXVII. 1), which in general have a regular and often a symmetrical disposition on the external surface (fig. CXXVII. 1). Sometimes, as in the water breathing annelides, these tufts form a fan-like expansion around the head; but at other times they are disposed in regular series along the whole extent of the body.

1. Respiratory tufts. 2. Artery and vein, supplying the respiratory apparatus. 3. Dorsal vessel.]

340. Instead of branchiæ in the form of ramified tufts, the ascending series of animals, namely, the higher crustacea, possess branchiæ composed of numerous, delicate, thin laminæ or leaves, divided from each other, yet placed in close proximity, like the teeth of a fine comb, whence this arrangement is termed pectinated. Over the blood-vessels of the system spread out on these delicate, fringed, pectinated leaves, the water is driven in constant streams.

341. Still higher in the scale, as in molluscous animals, an internal sac is formed to which are sometimes attached numerous tufts; but which at other times is itself plaited into beautifully disposed regular folds, crowded with blood-vessels and constantly bathed with fresh currents of water.

Trichoda showing the form and a frequent arrangement of Cilia.]

342. In all these water-breathing creatures, respiration is effected, either by the progressive motion of the body through the water, or by the creation of currents which bring fresh strata of the fluid into contact with the respiratory surfaces. Both objects are effected by the same instruments, namely, minute fibres having the appearance of fine hairs or bristles. These fibres which are called cilia, have in general an elongated, flattened, thin, and tapering form (fig. CXXVIII). Their number, position, and arrangement, are infinitely various. Sometimes, as in the poriferous animals, they are so minute that they cannot be rendered visible to the eye even by the microscope, although the evidence of their existence and action is indubitable. Sometimes they are of great size and strength, attached by distinct ligaments to the body and moved by powerful muscles, as in wheel animalcules. Sometimes, as in polypiferous animals, they are disposed around the orifice of the polypes or upon the sides of the tentacula, the instruments by which the animal seizes its prey. Sometimes they are symmetrically disposed in longitudinal series along the surface of the body, as in the Beroe pileus; at other times they are arranged in circles; whenever there are branchiæ, they are disposed around the margin of the branchial apertures, and always on the margins of the minute meshes which compose the branchiæ themselves.

343. In some cases the number of these cilia is immense. Each polype, for example, has usually twenty-two tentacula, and there are about fifty cilia on each side of a tentaculum, making two thousand two hundred cilia on each polype. As there are about one thousand eight hundred cells in each square inch of surface, and the branches of an ordinary specimen present about ten square inches of surface, we may estimate that an ordinary specimen of this zoophite presents more than eighteen thousand polypes, three hundred and ninety-six thousand tentacula, and thirty-nine million six hundred thousand cilia. But other species contain more than ten times these numbers. Dr. Grant has calculated that there are about four hundred million cilia on a single Flustra foliacea.

344. The motions of these cilia are regular, incessant, and when in full activity far too rapid to be distinguished by the eye even when assisted by the microscope. They are generally to be perceived only when their motions are comparatively feeble. They produce two effects. In animals capable of progressive motion, they transport the body through the water, while they constantly bring new strata of water into contact with the respiratory surface. In this case they are partly organs of locomotion, and partly organs subservient to respiration. On the other hand, in animals which are not capable of moving from place to place, they create currents by which the respiratory surface is constantly bathed with fresh streams of water. These currents are regular, constant, unceasing. Like some physical phenomena not depending on vitality, it is a continued stream as regular as the motions of rivers from their source to the ocean, or any other movements depending on the established order of things. Dr. Grant, to whom we are indebted for our knowledge of the true nature of these currents, as well as of the instruments by which they are effected, gives the following account of the observation which led to the discovery:—“I put,” says he, “a small branch of the spongia coalita, with some sea water into a watch-glass, under the microscope, and on reflecting the light of a candle through the fluid, I soon perceived that there was some intestine motion in the opaque particles floating through the water. On moving the watch-glass, so as to bring one of the apertures on the side of the sponge fully into view, I beheld, for the first time, the splendid spectacle of this living fountain, vomiting forth from a circular cavity an impetuous torrent of liquid matter, and hurling along in rapid succession opaque masses which it strewed everywhere around. The beauty and novelty of such a scene in the animal kingdom long arrested my attention, but after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction, or diminish in the slightest degree the rapidity of its course. I continued to watch the same orifice, at short intervals, for five hours, sometimes observing it for a quarter of an hour at a time, but still the stream rolled on with a constant and equal velocity.”

345. The simple expedients which have been described suffice for carrying on the function of respiration in the water-breathing invertebrata; but in creatures that possess a vertebral column, and the more perfect skeleton of which it forms a part, there is a prodigious advancement in the organization of the whole body, of the nervous and muscular systems especially, the organs of the animal, as well as in all the organs of the organic life. A corresponding development of the function of respiration is indispensable. Accordingly, a sudden and great development in the apparatus of this function is strikingly apparent in fishes, the lowest order of the vertebrata, in which the branchiæ, though still preserving the same form as in the animals below them, are large and complex organs. The branchiæ of fishes still consist of fringed folds of membrane disposed, as in the preceding classes, in laminæ or leaves (fig. CXXIX. 5); but there are now commonly four series of these leaves, on each side of the body, placed in close approximation to each other, the several leaves being divided into minute fibres, which are set close like the barbs of a feather, or the teeth of a fine comb (fig. CXXIX. 5). Each leaf rests either on a cartilaginous or a bony arch, which exactly resembles the rib of the more perfect skeleton, and performs a strictly analogous function; for these arches are capable of alternately separating from, and of approximating to, each other, and these alternate motions are effected by appropriate muscles. As these movements of separation or approximation take place, the branchiæ are either opened or closed, and their surface proportionally expanded or contracted. Upon these leaves (fig. CXXIX. 5) the veins (347) of the system (fig. CXXIX. 4) are spread out in a state of capillary division of extreme minuteness, forming a net-work of vessels of extreme tenuity and delicacy. So prodigiously is the surface increased for the expansion of these vessels by the leaf-like disposition of the branchiæ, that it is computed that the branchial surface of the skate is at least equal to the surface of the whole human body.

346. Through this extended surface the whole blood of the system must circulate, and every point of it must be unceasingly bathed with fresh streams of water. To generate the force necessary for the accomplishment of these objects, an increase of power must be communicated both to the circulating and to the respiratory apparatus. Neither the contractile power of the vessels by which in some of the simpler animals the nutritive fluid is put in motion, nor the contraction of the rudimentary heart by which in creatures somewhat higher in the scale a more decided impulse is given to the blood, are sufficient. A muscular heart, capable of acting with great power, is now constructed, which is placed in such a position as to enable it to propel with velocity the whole blood of the body through the myriads of capillary vessels that crowd every point of the surface of the branchial leaflets. To bring the water with the requisite degree of force into contact with this flowing stream, the apparatus of cilia is wholly inadequate. The water entering by the mouth, is driven with force, by the powerful muscles of the thorax, through apertures that lead to the branchial cavities. At the instant that the branchial leaves receive the currents of water through the appropriate apertures, the cartilaginous or bony arches which sustain the leaves, separate to some distance from each other, and to that extent expand the leaves and proportionally increase the surface exposed to the water: at the same time, the rush of water through the leaves unfolds and separates each of the thousand minute filaments of which they are composed, so that they all receive the full action of the fluid as it flows over them.

347. After the venous blood of the system has been thus exposed to the action of the respiratory medium, it is taken up by the vessels called the branchial veins (fig. CXXIX. 6), which for the reason assigned (372) are functionally arteries, as the branchial artery (fig. CXXIX. 4) is functionally a vein. The branchial veins uniting together form the great arterial trunk of the system, (fig. CXXIX. 7) by which the aërated blood is carried out to every part of the body.

348. But as if even this extent of apparatus were insufficient to afford the amount of respiration required by the system of the fish, the entire surface of its body, which in general is naked and highly vascular, respires like the branchiæ. Moreover, many fishes swallow large draughts of air, by which they aërate the mucous surface of their alimentary canal, which also is highly vascular; and still further, numerous tribes of these animals are provided with a distinct additional organ, a bag placed along the middle of the back filled with air. Commonly this air bag communicates with some part of the alimentary canal near the stomach, by means of a short wide canal termed the ductus pneumaticus, but sometimes it forms a simple shut sac without any manifest opening; at other times it is divided and subdivided in a perfectly regular manner, forming extended ramified tubes; while at other times its ramifications present the appearance of so many pulmonary cells. It is the rudiment of the complex lung of the higher vertebrata, and it assists respiration; although since in some tribes it contains not atmospheric air but azote, it is without doubt subservient to other uses in the economy of the animal.

349. In water-breathing animals, from the lowest to the highest, it is then manifest that a special apparatus is provided for, constantly renewing the streams of water that are brought into contact with their respiratory surface.

1. Integument or skin of the body. 2. Spiracula opening on the external surface of the skin. 3. Tracheæ, or air tubes, proceeding in form of radii from the spiracles to 4. the alimentary canal.]

350. It is the same in aërial respiration. In the simplest form of aërial respiration the apparatus consists of minute bags or sacs, placed commonly in pairs along the back, which open for the admission of the air on the external surface, by small orifices called spiracula or spiracles (fig. CXXX. 2), at the sides of the body. In the common earth-worm there are no less than one hundred and twenty of these minute air vesicles, each of which is provided with an external opening placed between the segments of the body. In the leech, the number is reduced to sixteen on each side, which open externally by the same number of minute orifices. Over the internal surface of these air vesicles the blood of the system is distributed in minute and delicate capillaries; and is capable of being aërated by whichever medium may pass through the external orifices, whether water or air.

351. In this simple apparatus is apparent the rudiment of the more perfect aërial respiration by the organs termed tracheæ, minute air tubes which ramify like blood-vessels through the body (fig. CXXX. 3). These air tubes open on the external surface by distinct apertures termed _spiracula_ or _spiracles_ (fig. CXXX. 2), which are commonly placed in rows on each side of the body (fig. CXXX. 2), with distinct prominent edges (fig. CXXX. 2), often surrounded with hairs; sometimes guarded by valves to prevent the entrance of extraneous bodies, and capable of being opened and closed by muscles specially provided for that purpose. These tubes, as they proceed from the spiracles to be distributed to the different organs of the body, often present the appearance of radii (fig. CXXX. 3), and when traced to their terminations are found to end in vesicles of various sizes and figures, but commonly of an elongated and oblong form. These minute vesicles, when examined by the microscope, are seen to afford still minuter ramifications, which are ultimately lost in the tissues of the body.

352. The tracheæ are composed of three tunics, the external dense, white and shining; the internal soft and mucous, between which is placed a middle tunic, dense, firm, elastic, and coiled into a spiral. By this arrangement the tube is constantly kept in a state of expansion, and is therefore always open to the access of air. A great part of the blood of the body, in the extensive class of creatures provided with this form of respiratory apparatus, including the almost countless tribes of insects, is not contained in distinct vessels, but is diffused by transudation through the several organs and tissues of the body. All the creatures of this class live in air, and possess great activity; they therefore require a high degree of respiration; yet they are commonly small in size, and often some portions of their body consist of exceedingly dense and firm textures; hence to have localized the function of respiration, by placing the seat of it in a single organ, would have been impossible, on account of the disproportionate magnitude which such an organ must have possessed; in this case it was easier to carry the air to the blood, than the blood to the air, and accordingly the air is carried to the blood, and, like the blood in creatures of higher organization, is diffused through every part of the system.

1. Spiracles. 2. Integument of one half of the body turned back. 3. Branchial organs. 4. Cells or pouches in which they are lodged. _a._ One of the respiratory organs removed and magnified, showing its resemblance to the branchial leaflets, and presenting the pectinated appearance described in the text.]

1. Trachea. 2. Vesicular lungs. 3. Stomach.]

353. The next advancement in the ascending scale is, by a step which obviously connects this higher class with the classes below and above it. It consists of distinct cells, termed pulmonic cavities (fig. CXXXI. 4), which communicate externally by spiracula (fig. CXXXI. 1), like tracheæ (351), but which are lined internally by a soft and delicate membrane plaited into folds, disposed like the teeth of a comb (pectinated) (fig. CXXXI. _a_), presenting a striking analogy to the structure of gills (345), and therefore called by the French writers pneumo-branchiæ. These cavities have the internal form of an aquatic organ, but they perform the function of air-breathing sacs. In scorpions (fig. CXXXI. 1) and spiders, this form of the apparatus is seen in its simplest condition; in the slug and snail it is more highly developed: for in these latter animals a rounded aperture, placed near the head, and guarded by a sphincter muscle, that alternately dilates and contracts, leads to a single cavity, which is lined with a membrane delicately folded, and overspread with a beautiful net-work of pulmonary blood-vessels.

354. Passing from this to the lowest order of the air-breathing vertebrata (fig. CXXXII.), the apparatus is perfectly analogous, but more developed. In the reptile, this air-breathing sac, which now constitutes a true and proper lung, instead of being simple and undivided, is formed by numerous septa, which traverse each other in all directions, into vesicles or cells (fig. CXXXII. 2), which proportionally enlarge the surface for the distribution of blood-vessels. In the Batrachian reptile, as the frog, salamander, newt, &c. (fig. CXXXII.), the vesicles, comparatively few in number, are of large size, and as thin and delicate as soap-bubbles. In the ophidian reptile, as the serpent, the sac is large and elongated, but divided only in the upper and back part into vesicles; while in the Saurian reptiles, as the crocodile, lizard, chamelion, &c., the sac is comparatively small, but subdivided into very minute vesicles, bearing a close analogy to the more perfectly organized lung of the higher animals.

355. In birds, the next order of vertebrata (fig. CXXXIII.), as in insects, the class of invertebrated animals which are formed for flight (352), the respiratory organs extend through the greater part of the body (fig. CXXXIII. 4). The lungs (fig. CXXXIII. 2), which still consist of a single pulmonic sac on each side (fig. CXXXIII. 2), are divided into cells, minute compared with those of the reptile, yet large compared with those of the quadruped; at the same time numerous air sacs, similar in structure to those of the lungs, but of larger size, are distributed over different parts of the body (fig. CXXXIII. 4), which communicate with the air cells of the lungs (fig. CXXXIII. 3); while of these larger sacs, several communicate also with the bones (fig. CXXXIII. 6), so as to fill with air those cavities which in other animals are occupied with marrow.

356. In the mammalia, the highest order of the vertebrata, respiration is less extended through the system, and is concentrated in a single organ, the lung, which, though comparatively smaller in bulk than in some of the lower classes, is far more developed in structure. The lung in this class consists of a membranous bag, divided into an immense number of distinct vesicles or cells, in the closest possible proximity with each other, yet not communicating, and presenting, from their minuteness, a vast extent of internal surface. This bag is confined to a distinct cavity of the trunk, the thorax (fig. CXXXIV.), completely separated from the abdomen by the muscular partition, the diaphragm (fig. CXXXIV. 10). This organ no longer sends down cells into the abdomen, nor membranous tubes into the bones; but is concentrated within the thorax along with the heart (fig. CXXXIV. 2, 3, 8). In all the orders of this class, the development and concentration of the organ are in strict proportion to the perfection of the general organization.

1. The Trachea. 2. The right lung. 3. The left lung. 4. Fissures, dividing each lung into, 5. Large portions termed lobes. 6. Smaller divisions termed lobules. 7. Pericardium. 8. Heart. 9. Aorta. 10. Diaphragm separating the cavity of the thorax from that of the abdomen.]

357. In man there are two pulmonary bags (fig. CXXXIV. 2, 3), of nearly equal size, which, together with the heart, completely fill the large cavity of the thorax (fig. CXXXIV.), their external surface being everywhere in immediate contact with the thoracic walls. One of these bags is placed on the right side of the body, constituting the right lung (fig. CXXXIV. 2), and the other on the left, constituting the left lung (fig. CXXXIV. 3). Each lung is divided by deep fissures, into large portions called lobes (figs. CXXXIV. 4, and CXXXV. 6), of which there are three belonging to the right, and two to the left lung. Each lobe is subdivided into innumerable smaller parts termed lobules (figs. CXXXIV. 6, and CXXXV. 6), while the lobules successively diminish in size until they terminate in minute vesicles that constitute the great bulk of the organ (fig. CXXXV. 8).

358. The complete centralization of the respiratory function which thus takes place in man, renders the apparatus exceedingly complex both on account of the expedients which are necessary to obtain the requisite extent of surface, in the small allotted space, and to bring into contact within that space the fluids that are to act on each other.

1. The larynx. 2. Trachea. 3. Right bronchus. 4. Left bronchus. 5. Left lung; the fissures denoted by the two lines which meet at 6, dividing it into three lobes, and the smaller lines on its surface marking the division of the lobes into lobules. 7. Large bronchial tubes. 8. Minute bronchial tubes terminating in the air cells or vesicles.]

359. The apparatus consists of a vessel to carry the air to the blood; a vessel to carry the blood to the air; an organ in which the air and the blood meet; and an organization by which both fluids are put in motion. The vessel that carries the air to the blood is the windpipe (fig. CXXXV. 1, 2); the vessel that carries the blood to the air is the pulmonary artery (fig. CXL. 7); the organ in which the blood and the air meet is the lung (fig. CXXXV. 5); the organization which puts the air in motion, is the structure of bones, cartilage and muscles, called the thorax (figs. CXLI. and CXLVI.), and the engine that communicates motion to the blood is the right ventricle of the heart (fig. CXL. 5).

360. The windpipe is a tube which extends from the mouth and nostrils to the lung (figs. CLIII. 1, 9, and CXXXV. 2, 5). It is attached to the back part of the tongue (fig. CLII. 2, 9), and passes down the neck immediately before the esophagus, or the tube which leads to the stomach (fig. CLIII. 9, 12).

361. In the different parts of its course the windpipe is differently constructed, performs different offices, and receives different names according to the diversity of its structure and function. The first division of it is called the larynx (fig. CXXXV. 1.), the second the trachea (fig. CXXXV. 2), the third the bronchi (figs. CXXXV. 3, 4, 7, and CXXXVII.), and the fourth the air vesicles or cells (figs. CXXXV. 8, and CXXXVIII. 2).

1. The os hyoides. 2. Thyroid cartilage. 3. Cricoid cartilage. 4. Arytenoid cartilages, separated from each other. 5. Epiglottis. 6. Opening of the glottis. 7. Termination of the cartilaginous rings of the trachea. 8. The ligamentous portion of the trachea. 9. Trachea laid open, showing its internal mucous surface and follicles, with the anterior portion of the cartilaginous rings appearing through it.]

362. The first portion of the windpipe called the larynx (figs. CXXXV. and CXXXVI), constitutes the organ of the voice. It is situated at the upper and fore part of the neck (fig. CLIII. 7, 9), immediately under the bone to which the root of the tongue, called the os hyoides (figs. CLIII. 6, and CXXXVI. 1), is attached. The larynx forms a very complex structure, and is composed of a variety of cartilages, muscles, ligaments, membranes, and mucous glands (fig. CXXXVI. 2, 3, 4, 5). At its upper part is a narrow opening of a triangular figure called the glottis (fig. CXXXVI. 6), by which air is admitted to and from the lung. Immediately above this opening is placed the cartilage, which obtains its name from its situation, _epiglottis_ (fig. CXXXVI. 5), which is attached to the root of the tongue (fig. CLIII. 6, 7), and which may be distinctly seen in the living body by pressing down the tongue.

363. The Epiglottis is highly elastic, and is an agent of no inconsiderable importance in respiration, deglutition, and speaking. In respiration it breaks the current of air which rushes to the lungs through the mouth and nostrils, and prevents it from flowing to the delicate air cells with too great a degree of force. During the action of deglutition the epiglottis is carried completely over the glottis (fig. CLIII. 6, 7, 8), partly because it is necessarily forced backwards, when the tongue passes backwards in delivering the food to the pharynx (fig. CLIII. 6, 7, 8, 10), partly because it is carried backwards by certain minute muscles which act directly upon it, and perhaps also partly in consequence of its own peculiar irritability. The moment the action of deglutition has been performed the epiglottis springs from the aperture of the glottis, partly by its own elasticity, and partly by the return of the tongue to its former position. During the act of speaking the column of air which is expelled from the lung, which rushes through the glottis, and which thus forms the voice, strikes against the epiglottis, and the voice becomes thereby in some degree modified.

View of the trachea, showing, first, the division of the tube into the right and left bronchus, and the subdivision of the bronchi into the bronchial tubes; and secondly, the membranous and cartilaginous tissues of which the organ is composed.]

364. The second portion of the windpipe termed the trachea (fig. CXXXV. 2), commences at the under part of the larynx (fig. CXXXV. 1), and extends as far as the third dorsal vertebra, opposite to which it divides into two branches which are termed the bronchi (fig. CXXXV. 3, 4, and CXXXVII.). One of these branches, called the right bronchus, goes to the right lung; the other branch, called the left bronchus, goes to the left lung (fig. CXXXV. 3, 4).

365. The trachea of man, like the tracheæ of the air-breathing insect (351), is composed of three tissues. These tissues differ essentially from each other in nature, and are widely different in form and arrangement. They consist of membrane, muscle, and cartilage.

366. The membranous portion of the human trachea consists of three coats, the cellular (fig. CXXXVII.), the ligamentous (fig. CXXXVI. 8), and the mucous (fig. CXXXVI. 9). From the cellular and ligamentous coats the tube receives its strength, and in some degree its elasticity; and the mucous coat constitutes the chief seat of the respiratory function. Between the ligamentous and mucous coats are placed two sets of muscular fibres; the first, the external set, passes in a circular direction around the tube; the second set, placed immediately beneath the circular, is disposed longitudinally, and collected into bundles. The office of the circular fibres is to diminish the calibre of the tube, and that of the longitudinal is to diminish its length.

367. As the tracheæ of the insect are kept constantly open for the free admission of air by their middle membranous tunic, dense, firm, elastic, and coiled into a spiral (351), so, for the accomplishment of the same purpose, there are placed between the membranous coats of the human trachea delicate rings of the more highly organized substance, cartilage (35). These cartilaginous rings amount in the entire course of the tube to sixteen or eighteen in number (fig. CXXXV. 2); each cartilage being about a line in breadth, and the fourth of a line in thickness. They never form complete circles, but only a large segment of a circle (fig. CXXXVI. 7); the circle is incomplete behind (fig. CXXXVI. 7, 9), because there the esophagus is in direct contact with the trachea (fig. CLIII. 9, 12), and instead of dense and firm cartilage, a soft and yielding substance is placed in this situation, in order that there may be no impediment to the free dilatation of the esophagus during the passage of the food.

368. The point at which the bronchi enter the substance of the lung is called the root of the lung (fig. CXXXV. 3, 4). As soon as the bronchi begin to divide and ramify within the lung each cartilage, instead of preserving its crescent shape, is divided into two or three separate pieces, which nevertheless are still so disposed as to keep the tube open. With the progressive diminution in the size of the bronchial branches, their cartilages become less numerous, and are placed at greater distances from each other, until at length as the bronchi terminate in the vesicles, the cartilages wholly disappear; and with the decreasing number and size of the cartilages, the thickness of the cellular, ligamentous, and muscular coats of the bronchi also lessens, until at the points where the cartilages disappear, the muscular and mucous tunics, now reduced to a state of extreme tenuity, alone remain. The essential constituent of the air vesicles, then, is the mucous membrane; but there is reason to suppose that the muscular tunic is likewise continued over these vesicles.

369. It has been stated that the tracheæ of the insect terminate in the different tissues of its body by minute vesicles of an oblong form. The termination of the bronchi in the human lung presents a strikingly analogous appearance. Malpighi, who with extraordinary talent and success devoted his life to the investigation of the minute structures of the various organs of the human body, represents the mucous membrane of the bronchial tubes as terminating in minute vesicles of unequal size: and Reisseissen, who has more recently resumed the inquiry and examined this structure with extreme care, agrees with Malpighi in stating that the bronchial tubes at their terminal points expand into minute, delicate, membranous vesicles of a cylindrical and somewhat rounded figure (fig. CXXXVIII. 2). The bronchial tubes do not divide to any great degree of minuteness (fig. CXXXVIII. 1), but terminate somewhat abruptly in the vesicles (fig. CXXXVIII. 2), which though minute are large enough to be visible to the naked eye (fig. CXXXVIII. 2). Viewed in connexion with the bronchial tubes at their terminal points, the vesicles present a clustered appearance, not unlike clusters of currants attached to their stem (fig. CXXXVIII. 2).

370. In the insect, for the reason assigned (351), these vesicles are diffused over the system, aërating every point of the body; in man they are concentrated in the lung; yet by their minuteness, and by the mode in which they are arranged, they present in the small space occupied by this organ, so extended a surface that Hales, representing the size of each vesicle at the 100dth part of an inch in diameter, estimates the amount of surface furnished by them collectively at 20,000 square inches. Keil estimating the number of the vesicles at 174,000,000, calculates the surface they present, at 21,906 square inches. Leiberkuhn at 150 cubic feet; and, according to Monro, it is thirty times the surface of the human body.

1. The trachea. 2. The right and left bronchus; the left bronchus showing its division into smaller and smaller branches in the lung, and the ultimate termination of the branches in the air vesicles. 3. Right auricle of the heart. 4. Left auricle. 5. Right ventricle. 6. The aorta arising from the left ventricle, the left ventricle being in this diagram concealed by the right. 7. Pulmonary artery arising from the right ventricle and dividing into, 8. The right, and 9. The left branch. The latter is seen dividing into smaller and smaller branches, and ultimately terminating on the air vesicles. 10. Branches of one of the pulmonary veins proceeding from the terminations of the pulmonary artery on the air vesicles, where together they form the net-work of vessels termed the Rete Mirabile. 11. Trunk of the vein on its way to the left auricle of the heart. 12. Superior vena cava. 13. Inferior vena cava. 14. Air vesicles magnified. 15. Blood-vessels distributed upon them.]

371. Such is the structure of the vessel that carries the air to the blood, and such is the mode of its distribution.

The vessel that conveys the blood to the air is the pulmonary artery, the great vessel which springs from the right ventricle of the heart (fig. CXL. 5).

The pulmonary artery soon after it issues from the right ventricle of the heart divides into two branches (fig. CXL. 7, 8, 9), one for each lung (fig. CXL. 8, 9). Each branch of the pulmonary artery as soon as it enters its corresponding lung (fig. CXL. 9) divides and ramifies through the organ in a manner precisely similar to the bronchial tubes. Every branch of the artery is in contact with a corresponding branch of the bronchus (fig. CXL. 2), divides as it divides, and accurately tracks its course throughout (fig. CXL. 2), until the ultimate divisions of the artery at length reach the ultimate vesicles of the bronchus (fig. CXL. 2, 10), upon the delicate walls of which the capillary arteries rest, expand, and ramify, forming a net-work of vessels, so complex that the anatomist who first observed it, named it the _Rete Mirabile_, the wonderful net-work, and it is still called the _Rete Mirabile Malpighi_, or the _Rete Vasculosum Malpighi_ (fig. CXL. 2, 9, 10).