400 TEXT-BOOK OF PHYSIOLOGY. 



known as the bronchial sound. It is especially well heard between the 

 scapulae above the fourth thoracic vertebra. This sound is produced 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 uncer- 

 tain. On applying the ear to almost any portion of the chest- wall, but especially 

 to the infrascapular area, there is heard during both inspiration and expiration 

 a delicate, sighing, rustling sound, which from its supposed seat of origin, 

 the air- vesicles or air-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 friction of the air in the alveolar passages. 



THE CHEMISTRY OF RESPIRATION. 



The general metabolic process as it takes place in the tissues involves 

 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 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 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 exchanges between blood and tissues constitute internal respiration, 

 in contradistinction to the thoracic movements by which the air is brought 

 into relation with the blood, and which constitute external respiration. The 

 transfer of the oxygen by the blood from the interior of the lungs to the tissues, 

 and of the carbon dioxid from the tissues to the interior of the lungs, is the 

 outcome of a series of physical and chemic changes which are related to the 

 exchange of gases between the air in the lungs and the blood, on the one 

 hand, and between the blood and tissues, on the other. 



In consequence of the many and complex chemic changes which attend 

 these gaseous exchanges, there arise changes in composition of: 



1. The air breathed. 



2. The blood, both arterial and venous. 



3. The tissue elements and the lymph by which they are surrounded. 



The investigation of the nature of these changes, the mechanism of their 

 production, and their quantitative relations constitute the subject-matter of 

 the chemistry of respiration. 



CHANGES IN THE COMPOSITION OF THE AIR. 



Experience teaches that the air during its sojourn in the lungs undergoes 

 such a change in composition that it is rendered unfit for further breathing. 

 Chemic analysis has shown that this change involves a loss of oxygen, a gain 

 in carbon dioxid, watery vapor, and organic matter. For the correct under- 

 standing of the phenomena of respiration it is essential that not only the 

 character but the extent of these changes be known. This necessitates an 

 analysis of both the inspired and expired airs, from a comparison of which 

 certain deductions can be made. 



