Now there are some remarkable features worth pointing out in the structure of the windpipe and bronchial tubes. Unless some provision was made to the contrary, the tubes, even the largest of them, would readily collapse, their walls would fall together with the slightest pressure, and the fair-way for the passage of air would be closed. The walls are, however, stiffened by pieces of cartilage (gristle). In the windpipe the pieces of car tilage are C-shaped, the deficient part being behind, resting on the gullet. In Fig. 149, w points to one of the cartilaginous rings, and in Fig. 155 (p. 354) is seen how they terminate behind. In the bronchial tubes plates of car tilage are disposed in the circumference of the walls, and act like the rings of the windpipe, keeping the tubes open. In the smallest tubes, however, the cartilage is absent, and the tubes become liable to spasmodic closure. In the windpipe the defective portion of the rings behind is closed by a band of muscular fibres of the involuntary kind 113), while in the bronchial tubes there is a middle coat formed of a continuous layer of involuntary muscular fibres disposed round the walls. Both wind pipe and bronchial tubes have elastic fibres running the length of the tubes. The inner surface is lined by mucous membrane con tinuous, through the larynx, with that of the throat, mouth, and nostrils. An inflammation attacking the throat is thus apt to travel along the membrane downwards into the windpipe and bronchial tubes, leading to cough, and perhaps to bronchitis. This mucous mem brane is covered by layers of epithelial cells, those on the surface being provided with the hair-like processes called cilia, depicted in Fig. 6, p. 54. The mucous membrane is provided with glands, which, by their secretion, keep the membrane moist. These anatomical details are interesting because of the purposes which the structures are fitted to serve. As already pointed out, the cartilage keeps the tubes open. The elastic tissue confers elasticity on the tubes, permits them to stretch readily with the movements of the neck, &c., and to return to their usual condition when the stretching is over. The cilia of the epithelium keep up a constant waving movement, all in the same direction, namely upwards, and thus any excessive secretion or defluxion is gently urged upwards to the top of the windpipe, from which it is expelled by a cough. Material is thus prevented from passing downwards and accumulating in the small tubes, so that they are in ordinary circumstances kept clear. It is believed that the muscular layer, specially of the smaller tubes, acts in a similar way, sweep ing upwards by its contractions matter to be expelled. An eminent physician has, on this account, called them "scavenger muscles." It may be mentioned here that this muscular coat, in unhealthy conditions, may by its ex cessive contraction produce a serious difficulty in breathing. If muscular spasm occurs in the smallest tubes, unprovided with cartilage, it may close the tubes, prevent the passage of air, and so occasion severe difficulty of breathing. This spasm would be produced, among other things, by excessive nervous action, and is believed to be the cause of the difficulty of breathing in nervous asthma. But, again, it should be noticed that if one attempted to breathe an irritating gas the muscular coat would be stimulated to contract, would bar the way to the entrance of the hurtful gas, and so be of great benefit.
It has been noted that the smallest bron chial tubes have no plates of gristle in their walls. They lose also the hair-like processes of the epithelial cells, which are reduced to one fine layer.
The an earlier paragraph of this section the lungs were, for the moment, represented by two bags or sacs communicat ing by means of tubes with the external air. This would almost be a proper description of the lungs of some of the lower animals, if we add to it that the sacs have depressions in the inner surface of their walls resembling the cells of a honeycomb. In the lungs of the higher
animals and of man the structure is touch more complicated, although built up, so to speak, on the type of the elementary structure just noticed. If one of the smallest bronchial tubes be traced to its extremity it is found that it leads into a passage wider than itself, and that from that passage there open out on all sides honeycomb-like cells. This is repre sented in outline in ? of Fig. 151, where b indi cates the termination of the bronchial tube, h h h the passage into which it leads, and cc c the cells opening off the passage. The cells are called air-cells or alveoli (Latin, alveolus, a small cavity), and the passage is the alveolar passage. The whole arrangement of passage and air-cells springing from the termination of a bronchial tube is called an :afundibulum, because of its widening out from the place where it arises from the bronchial tube. It is also called an ultimate lobule. in of Fig. 151 shows two such infundibula from the lungs of a newly-born child (magnified twenty times) as they appear when not opened up, as they are in 1. Now each such ultimate lobule of a human lung is a very small miniature of the whole lung of some of the lower animals. So that a whole human lung is a kind of assem blage of miniature lungs of the type of the lower forms. For several infundibula are grouped or packed together, and thus form a lobule, larger than the ultimate lobule, from a quarter to half an inch in diameter, and bound by connective tissue. Several lobules are bound together in a similar way to form a lobe of the lung. The right lung has three such lobes, and the left two.
The walls of the air-cells are very thin, con sisting of delicate elastic and connective tissue, and lined inside by flat transparent cells. In the connective tissue run capillary vessels be longing to the pulmonary artery and veins (p. 301). Now if Fig. 151, n, is attentively con sidered it will be seen that these thin-walled vessels running in the connective tissue are surrounded on all sides by air-cells, so that the blood flowing through them is only sepa rated by the thin walls of the vessels, and by the delicate tissue of the air-cells, from the air which these cells contain. The deep signi ficance of this will be apparent immediately.
It need only be noted now that such an arrangement is perfectly adapted for exchanges taking place between the blood in the capil laries and the air inn the air-cells.
Each lung is surrounded by an investing membrane—the pleura. Like the investing membrane of the heart (p. 298) the pleura of each lung is in a double layer. One layer closely envelops the lung and at the root of the lung is folded back on to the wall of the chest cavity, of its own side, which it lines. The two layers thus form between them a shut sac, a serous cavity. No cavity exists, however, in health, but the two layers glide on one another, and the inner sun-face secretes a slight amount of serous fluid to prevent friction. It is in flammation of this membrane that is called pleurisy (p. 359).
Through the root of the lung vessels pass to and from the lung. The pulmonary artery enters carrying blood from the right side of the heart to the capillaries of the air-cells, to be purified by contact with the air the cells contain. The pulmonary veins leave the lung by the root to pass to the left side of the heart, carrying the purified blood. Branches from the great artery--the aorta,—called bronchial arteries, enter the lungs at the root, carrying pure blood to be distributed over the bronchial tubes and among the connective tissue of the lungs, to maintain their nourishment. Lym phatic vessels also pass out by the root to join the thoracic duct (p. 278). For lymphatic channels are distributed through the lungs over the bronchial tubes, and are in direct communication with the air-cells. It is by means of the lymphatic vessels that matters are absorbed from the lungs, and afterwards may be poured into the current of the circula tion, after passing through glands.