CHAPTER V.

RESPIRATION.

THE maintenance of animal life necessitates the continual absorption of oxygen and excretion of carbonic acid; the blood being, in all animals which possess a well developed blood-vascular system, the medium by which these gases are carried. By the blood, oxygen is absorbed from without and conveyed to all parts of the organism; and, by the blood, carbonic acid, which comes from within, is carried to those parts by which it may escape from the body. The two processes,-absorption of oxygen and excretion of carbonic acid, are complementary, and their sum is termed the process of Respiration.

In all Vertebrata, and in a large number of Invertebrata, certain parts, either lungs or gills, are specially constructed for bringing the blood into proximity with the aërating medium (atmospheric air, or water containing air in solution). In some of the lower Vertebrata (frogs and other naked Amphibia) the skin is important as a respiratory organ, and is capable of supplementing, to some extent, the functions of the proper breathing apparatus; but in all the higher animals, including man, the respiratory capacity of the skin is so infinitesimal that it may be practically disregarded.

Essentially, a lung or gill is constructed of a fine transparent membrane, one surface of which is exposed to the air or water, as the case may be, while, on the other, is a network of bloodvessels, the only separation between the blood and aërating medium being the thin wall of the blood-vessels, and the fine membrane on one side of which vessels are distributed. The difference between the simplest and the most complicated respiratory membrane is one of degree only.

The various complexity of the respiratory membrane, and the kind of aërating medium, are not, however, the only conditions which cause a difference in the respiratory capacity of different animals. The number and size of the red blood-corpuscles, the mechanism of the breathing apparatus, the presence or absence of a pulmonary heart, physiologically distinct from the systemic, are, all of them, conditions scarcely second in importance.

In the heart of man and all other Mammalia, the right side from which the blood is propelled into and through the lungs may be termed the "pulmonary" heart; while the left side is "systemic" in function. In many of the lower animals, however, no such distinction can be drawn. Thus, in Fish the heart propels the blood to the respiratory organs (gills); but there is no contractile sac corresponding to the left side of the heart, to propel the blood directly into the systemic vessels.

It may be well to state here that the lungs are only the medium for the exchange, on the part of the blood, of carbonic acid for oxygen. They are not the seat, in any special manner, of those combustion-processes of which the production of carbonic acid is the final result. These occur in all parts of the bodymore in one part, less in another: chiefly in the substance of the tissues.

The Respiratory Passages and Tissues.

The object of respiration being the interchange of gases in the lungs, it is necessary that the atmospheric air shall pass into them and be expelled from them. The lungs are contained in the chest or thorax, which is a closed cavity having no communication with the outside, except by means of the respiratory passages. The air enters these passages through the nostrils or through the mouth, thence it passes through the larynx into the trachea or windpipe, which about the middle of the chest divides into two tubes or bronchi, one to each (right and left) lung.

The Larynx is the upper part of the passage which leads exclusively to the lung: it is formed by the thyroid, cricoid, and arytenoid cartilages (fig. 144), and contains the vocal cords, by the vibration of which the voice is chiefly produced. These vocal cords are ligamentous bands attached to certain cartilages capable of movement by muscles. By their approximation the cords can entirely close the entrance into the larynx ; but under ordinary conditions, the entrance of the larynx is formed by a more or less triangular chink between them, called the rima glottidis. Projecting at an acute angle between the base of the tongue and the larynx to which it is attached, is a leaf-shaped cartilage, with its larger extremity free, called the epiglottis (fig. 144, e). The whole of the larynx is lined by mucous membrane, which, however, is extremely thin over the vocal

cords. At its lower extremity the larynx joins the trachea.* With the exception of the epiglottis and the so-called cornicula laryngis, the cartilages of the larynx are of the hyaline variety.

Structure of the Epiglottis.-The supporting cartilage of the epiglottis is composed of yellow elastic cartilage, enclosed in a fibrous sheath (perichondrium), and covered on both sides with mucous membrane. The anterior surface, which looks towards the back of the tongue, is covered with mucous membrane, the basis of which is fibrous tissue, elevated towards both surfaces in the form of rudimentary papillæ, and covered with several layers

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of squamous epithelium. In it ramify capillary blood-vessels, and in its meshes are a large number of lymphatic channels. Under the mucous membrane,

in the less dense fibrous tissue

of which it is composed, are a number of tubular glands. The posterior or laryngeal surface of the epiglottis is covered by a mucous membrane, similar in structure to that on the other surface, but its epithelial coat is thinner, the number of strata of cells are less, and the papillæ few and less distinct. The fibrous tissue which constitutes the mucous membrane is in great part of the adenoid variety, and is here and there collected into distinct masses or follicles. The glands of the posterior surface are smaller but more numerous than those of the other surface. In many places the glands which are situated nearest to the perichondrium are directly continuous through apertures in the cartilage with those on the other side, and often the ducts of the glands from one side of the cartilage pass through and open upon the mucous surface of the other side. Taste goblets have been found in the epithelium of the posterior surface of the epiglottis, and in several other situations in the laryngeal mucous membrane.

Fig. 144.-Outline showing the general form of the larynx, trachea, and bronchi, as seen from before. h, the great cornu of the hyoid bone; e, epiglottis; t, superior, and ', inferior cornu of the thyroid cartilage; c, middle of the cricoid cartilage; tr, the trachea, showing sixteen cartilaginous rings; b, the right, and b', the left bronchus. (Allen Thomson.) .

The Trachea and Bronchial Tubes.-The trachea or windpipe extends from the cricoid cartilage, which is on a level with

the fifth cervical vertebra, to a point opposite the third dorsal vertebra, where it divides into the two bronchi, one for each lung

diminish, when required, the calibre of the trachea by approximating the ends of the cartilages. Outside these are a few longi