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.

Section of small portion of Lung, in which are seen a bronchial tube.

Fig. 145. Section of small portion of Lung, in which are seen a bronchial tube with its plicated lining mucous membrane in the centre, and the large blood vessels at the sides cut across. Loose areolar tissue and numerous lymphatics surround the large vessels and separate them from the lung tissue.

Muscles 01 Larynx, viewed from above.

Fig. 146. Muscles 01 Larynx, viewed from above.

Th. Thyroid cartilage. Cr. Cricoid cartilage. V. Edges of the vocal cords. Ary. Arytenoid cartilages. Th. A. Thyro-arytenoid muscle, c. a. I. Lateral crico-arytenoid muscle. c.a.p. Posterior crico-arytenoid muscle. Ar.p. Posterior arytenoid muscle.

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.