CIRCULATION, RESPIRATION AND EXCRETION 133 



cular partition, convex toward the 

 chest, the diaphragm. The air 

 enters the lungs through the 

 trachea, or windpipe, from the 

 mouth and nostrils. The trachea, 

 after reaching the chest, divides 

 into two branches, or bronchi, 

 one leading to the right and one 

 to the left lung. Each bronchus 

 subdivides repeatedly into a 

 multitude of fine tubes, the 

 smallest of which are called bron- 

 chioles (little bronchi), each of 

 which finally ends in an alveolus, 

 the inner surface of which is much , FlG - " ~ AlveoU of lun s- ( wil - 



111. , ckens, Form und Leben der Land- 



increased by being arranged in wirthschaftiichen Hausthiere.) 

 the form of pits or air cells. 



In Fig. 21, c represents a bronchiolus, aa two alveoli and bb 

 the air cells. Figure 22 shows diagrammatically on a large 

 scale a cross section of two alveoli. 



FIG. 22. Section of two alveoli. (Hough and Sedgwick, The Human 

 Mechanism.) 



