Page:First course in biology (IA firstcourseinbio00bailrich).pdf/541

 is to draw the ends together at times (for instance, during coughing) and reduce the size of the tube. The function of the hoops of cartilage is to keep the windpipe open at all times. If it should be closed by pressure, life might be lost. These rings of cartilage may be felt in the neck.

The lower end of the trachea is just behind the upper end of the breastbone; there it divides into two large tubes. These subdivide into a great number of smaller branches called bronchial tubes. Cartilage is found in the walls of all but the smallest of the tubes. The subdivision continues, somewhat like the branching of a tree, until the whole lung is penetrated by bronchial tubes. Each tiny tube finally ends in a wider funnel-shaped chamber called a lobule (Fig. 70), into which so many dilated sacs, called air cells, open, that the walls of the terminal chamber or lobule may be said to consist of tiny cups, or air cells, placed side by side. The lobules, or clusters of air cells, are chiefly near the surface of the lung. (The word "cell" is here used in its original sense to denote a cavity or chamber, and not in the sense of a protoplasmic cell.)

The air cells are elastic and enlarge by stretching as the chest expands; hence, the cells

(enlarged 30 diameters). Air sacs in white spaces. Dark lines are capillaries. (Peabody.)