358 



TEXT-BOOK OF PHYSIOLOGY. 



INSp. 



the object being to prevent a too rapid vitiation of the air. In- 

 spiration causes the lever to descend, expiration to ascend. Previous 

 graduation of the apparatus is necessary to determine the volumes 



breathed. A graphic record of 

 the volume changes is shown in 

 Fig. 176. 



Respiratory Sounds. On 

 applying the ear over the trachea 

 and bronchi there is heard dur- 

 ing both inspiration and expira- 

 tion a well-defined sound, loud, 

 harsh, and blowing in character, 

 which from its situation is known 

 as the bronchial sound. It is es- 

 pecially well heard between the 

 scapulae above the fourth dorsal 

 vertebra. This sound is pro- 

 duced in the larynx, for with its 

 separation from the trachea .the 

 sound disappears. The cause 

 of the sound is to be found in 

 the narrowing of the air-passage 



at the level of the vocal membranes, though the mechanism of its 

 production is uncertain. On applying the ear to almost any portion 

 of the chest- wall, but especially to the infrascapular area, there is 

 heard with each inspiration a delicate, sighing, rustling sound, which 

 from its supposed seat of origin, the air- vesicles or -cells, is known 

 as the vesicular sound. This sound is supposed to be due to the 

 sudden expansion of the air-cells during inspiration and to the fric- 

 tion of the air in the alveolar passages. 



FIG. 176. REPRESENTING THE VOLUME 

 CHANGES OF THE THORAX AND 

 LUNGS (DIAGRAMMATIC). 



THE CHEMISTRY OF RESPIRATION. 



The general nutritive process as it takes place in the tissues in- 

 volves the assimilation of oxygen and the evolution of carbon dioxid. 

 The former is the first, the latter the last, of a series of chemic changes 

 the continuance of which is essential to the maintenance of all life 

 phenomena. A constant supply of oxygen and an equally constant 

 removal of carbon dioxid are necessary conditions for tissue activity. 

 The respiratory movements constitute the means by which the oxygen 

 of the air is brought into, and the carbon dioxid expelled from, the 

 lungs into the surrounding air. The blood is the medium by which 

 the oxygen is transported from the lungs to the tissues and the carbon 

 dioxid from the tissues to the lungs. 



The exchanges between blood and tissues constitute internal 



