RESPIRATION'. 91 



with the ends of the rubber tubes on the corresponding sides of the 

 bag. Slip a short piece of the rubber tube on the end of the glass 

 tube, and when the balloon is inflated shut the air in by means of a 



CILIA | 1 BRONCHIAL TUBE. 



Fig. 42. Minute Structure of the Lungs, showing Air Vesicles 

 and Capillaries. 



pinchcock. The balloon represents an air vesicle, the glass tube a 

 bronchial twig, the blue tube a subdivision of the pulmonary artery, 

 the netting the capillaries around the vesicle, and the red tube one of 

 the branches of the pulmonary veins. 



The Movements of Respiration. The process of res- 

 piration consists of two acts, inspiration and expiration. 



Two Active Forces in Inspiration. In inspiration 

 the principal active forces in the body are, first, the dia- 

 phragm ; and, second, the muscles which elevate the ribs. 



Work of the Diaphragm in Inspiration. The dia- 

 phragm is a muscle, and when its fibers shorten, the dia' 

 phragm is pulled down. In moving down it presses on 

 the abdominal organs, and makes the abdomen protrude 

 laterally and ventrally. This lowering of the diaphragm 

 increases the space in the chest ; the air already in the 



