gases can find their way to and from the tissues and external air respectively.

Here, as in the case of the nutritive materials, the blood acts as the carrier. The pulmonary half of the circulation is devoted to the gas interchange between the blood and the atmosphere, and is sometimes spoken of as external respiration. The gas interchange between the blood and the tissues takes place in the general systemic capillaries, and has, therefore, been spoken of as the internal or tissue respiration.

In mammalia the pulmonary apparatus is so far perfected that all the necessary gas interchange can be carried on by the lungs, and the respiratory influence of the external skin or the mucous passages may be regarded as insignificant. But it should be remembered that, whenever the blood is in close relation to oxygen, as in the case of swallowed air, the oxygen is soon absorbed · by the blood.

In some of the lower animals the cutaneous surface aids very materially in respiration; for example, frogs can live by this cutaneous respiration alone for an almost indefinite time.

The change in the lungs consists in (1) oxygen being taken from the atmospheric air by the blood and (2) carbon dioxide being given off from the blood to the air. In the capillaries, on the other hand, the blood takes the carbon dioxide from the tissues, and yields to them a great portion of its oxygen.

RESPIRATORY MECHANISM IN LOWER ANIMALS.

In the lowest class of animals (e. g., amoeba), we find no special organs for the purpose of respiration, the gas interchange being sufficiently provided for by the exposure of the general surface of their bodies to the medium in which they live, namely, water. All higher animals have some special apparatus for the pur

The composition of the atmosphere is everywhere remarkably constant, in spite of its oxygen being used up by living beings. It consists of-

pose of respiration. This apparatus has always the same essential object, that of exposing their tissues to a medium containing oxygen, and of removing the carbonic acid gas.

In some of the invertebrate animals it suffices to distribute the medium containing oxygen throughout the tissues of the animal by means of tubes. Thus in the Echinodermata a water vascular system exists which seems to carry on the function of respiration. A somewhat similar method of distribution of oxygen takes place in arthropoda, in which delicately branching open tubes (tracheæ) distribute air to the tissues of the animal's body.

FIG. 144.

When more active changes occur in the tissues there is always a perfect blood vascular system. The blood is invariably used as the distributing and collecting agent of the gases in the tissues, and by flowing through some special organ exposed to the surrounding medium it ensures the gas interchange between the body and the outer world. These organs are formed on two general types: (1) external vascular fringes and (2) internal vascular

sacs.

Diagram of the Respiratory Organs. The windpipe leading down from the larynx is seen to branch into two large bronchi, which subdivide after they enter their respective lungs.

Animals living in water have commonly the external fringe arrangement (gills), while those living in air have sacs (lungs). Some animals (frogs, toads, etc.) have gills in the early stages of their life, and lungs when they are more fully developed. In frogs and serpents the lungs are simple sacs, with the inner surface increased by folds of the lining membrane, which gives it a honeycomb appearance; into each sac opens one of the divisions of the air tube. In crocodiles the air tubes divide into several branches, which open into a series of anfractuous, vascular recesses communicating one with another.

In birds wide bronchial tubes pass through the lung tissue to reach large air cavities. The walls of the tubes are studded with the openings of innumerable air cells lined with capillary blood vessels. The terminal air cavities are not vascular as in the mammalian lung.

STRUCTURE OF THE LUNG AND AIR PASSAGES.

The respiratory apparatus of mammals consists of (1) vascular sacs filled with air, known as the lung alveoli; (2) channels

by which these sacs are ventilated-the air passages; (3) motor arrangements, which carry on the ventilation of the lungs-the thorax.

1. The lungs are made up of innumerable minute cavities (alveoli), with thin septa springing from the inner surface so as to divide the space into several compartments or air cells. Each of these cavities forms a dilatation on the terminal twig of a branching bronchus, and may be regarded as an elementary lung. The aggregate of these cavities, and the branches of the

air passages and vessels distributed to them make up the structure of the lung.

The walls of the cavities are formed chiefly of fine elastic fibres, and the surface is lined with exceptionally delicate and thin-celled epithelium. Supported in the delicate framework of elastic and connective tissue is the remarkably close-set network of capillaries, in which the blood is exposed to the air. The delicate wall of the vessel and the thin body of the epithelial lining cell are the only structures interposed between the blood and the air. Pleura.-The external surface of the lungs is invested by a

Transverse section of part of the wall of a medium-sized bronchial tube.

X 30.

a. Fibrous layer containing plates of cartilage, glands, etc. b. Coat composed of unstriated muscle. c. Elastic sub-epithelial layer. d. Columnar ciliated epithelium.

serous membrane, the pleura, which is reflected to the wall of the thorax from the roots of the lungs, and completely lines the cavity in which they lie. Thus the lungs are only attached to the thorax where the air passages and great vessels enter, the rest of their surface being able to move over the inner surface of the thorax, and to retract from the chest wall if air be admitted into the pleural sac.

2. The air passages are kept permanently open during ordinary breathing by the elasticity of their tissues. The trachea and bronchi have special cartilaginous springs for the purpose. These

are closely attached to the fibro-elastic tissues which complete the general foundation of the walls of the tubes. The air passages are throughout lined with ciliated columnar epithelium, which, at the entrance to the infundibula, loses its cilia, and is converted into a single layer of flattened cells.

The air passages are supplied with muscle tissue of different kinds. Besides the ordinary striated muscles that control the opening of the anterior and posterior nares and pharynx, a special set surrounds the upper part of the larynx, and is capable

Section of a portion of Lung Tissue, showing part of a very small bronchus
cut across. (F. E. Schultze.)

a. Fibrous layer containing blood vessels. b. Layer of unstriated muscle. c. Layer of elastic fibres. d. Ciliated epithelium.

of completely closing the glottis, and thus shutting off the lung cavities, and proper air passages from the outer air. (Fig. 146.)

In the trachea a special muscle exists which can narrow the windpipe by approximating the extremities of the C-shaped springs that normally preserve its patency.

In the bronchial tubes a large quantity of smooth muscle cells exist, for the most part arranged as a circular coat, which is best developed in the small tubes (Fig. 148, b). As we pass from the large to the smaller bronchi the walls become thinner and less rigid, and the cartilaginous plates and fibrous tissue gradually