112 



RESPIRATION 



recti, contract and press the abdominal contents upward against the 

 diaphragm. The pectoral muscles also aid in forced respiration, as do, 

 indeed, at times, in one way or another, nearly all the muscles attached 

 to the thorax. 



THE NERVES. The nerves employed in respiration are a very impor- 

 tant part of the mechanism. It is necessary that the parts of the 

 apparatus should work perfectly together, and essential that the respira- 

 tory function should be adapted to the many changing conditions of the 



FIG. 66 



Internal. 



External. 



True respiration. Internal respiration is the interchange between the tissue-cells and the blood. 

 External respiration is the interchange between the blood and the lung-alveoli. 



rest of the organism. The afferent nerves of respiration pass from 

 different parts of the respiratory tract to the breath-center in the medulla 

 oblongata. Among these are the fifth cranial (the trigeminal), the first 

 (olfactory), the laryngeal, glosso-pharyngeal, and the vagus. Everyone 

 is aware how easily a sneeze may be produced by a sharp stimulation of 

 the nostrils, and inhibited by pressure on the upper lip. The afferent 

 branches of the vagal nerves probably ramify in the walls of the lungs of 



