MOVEMENTS OF RESPIRATION. 113 



tube a bronchial twig, the blue tube a subdivision of the pul- 

 monary artery, the netting the capillaries around the vesicle, 

 and the red tube one of the branches of the pulmonary veins. 



That the lungs do not collapse after death may be shown 

 in a cat or rabbit. Open the abdominal cavity, and pull back 

 the liver and stomach. The pink lungs may be seen through 

 the thin diaphragm. Pull the diaphragm back to see that the 

 lungs follow it, keeping in contact with it all the time. Now 

 lightly puncture the diaphragm on one side. The lung on 

 this side collapses, but the lung of the other side remains ex- 

 panded, showing that the two pleural cavities are separate. 



The Movements of Respiration. The process of res- 

 piration consists of two acts, inspiration and expiration. In 

 inspiration the principal active forces in the body are, first, the 

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



The diaphragm is a muscle, and when its fibers shorten, 

 the diaphragm is pulled down. In moving down it presses on 

 the abdominal organs, and makes the abdomen protrude later- 

 ally and ventrally. This lowering of the diaphragm increases 

 the space in the chest ; the air already in the chest expands 

 to fill this greater space. When expanded it exerts less pres- 

 sure than before, and the air outside, having greater pressure, 

 enters till equilibrium is produced. The air enters through 

 the trachea, presses on the inside of the elastic lungs, and 

 makes their bases 'extend, following the diaphragm in its de- 

 scent. The bases of the lungs remain in contact with the 

 upper surface of the diaphragm all the time. 



Certain muscles of the chest wall elevate the ribs and ster- 

 num. This act widens the chest; and the air, as before, 

 presses in through the open trachea, and keeps the sides of 

 the lungs in contact with the inner surfaces of the chest 

 walls. 



Inspiration requires considerable effort, because the dia- 



