RESPIRATION 169 



It is not possible to make anything like an accurate 

 estimate of the number of alveoli in the lungs ; 725,000,000 is 

 a figure arrived at by measuring the average cubic capacity 

 of an alveolus, and comparing it with the total cubic capacity 

 of the lungs. Each alveolus supports some forty or fifty 

 capillary vessels. The superficial area of vascular membrane 

 exposed is placed at 90 square metres, or about 100 times the 

 area of the skin. Figures such as these convey very little 

 meaning, but they help one to realize the magnitude of the 

 provision made for the aeration of the blood. 



Pneumonia is a condition in which the lining of the air- 

 chambers is inflamed, usually, possibly always, owing to the 

 entrance of bacteria. Lymph exudes through the walls of the 

 alveoli. Epithelial scales flake off. Pus-cells (dead leucocytes) 

 accumulate in the air-chambers. Respiration is curtailed, and 

 dyspnoea results. After a time, if the case progresses favour- 

 ably, " resolution," as it is technically termed, begins to occur. 

 The exuded substances are either expectorated or absorbed, 

 and the lung-tissue returns to a normal condition. 



Here a few words may be devoted to respiratory sounds. 

 Spirare means to sigh. Breathing received the name by which 

 it is known in physiology from the sound which accompanies 

 the exit of air from the nostrils. Since the introduction of 

 auscultation as a means of ascertaining the condition of the 

 lungs, other sounds, not heard until the ear or a stethoscope is 

 placed against the chest, have acquired great importance. 

 These sounds, termed " murmurs," may be divided into two 

 classes, (a) When the ear is placed against the windpipe, or in 

 the middle of the back between the shoulder-blades, a murmur 

 is audible, due to the movement of air through the larynx. 

 If the larynx, the trachea, or the bronchi contain mucus, it is 

 a harsh, rough, bubbling, or crackling sound. It accompanies 

 both inspiration and expiration, (b) A softer, more delicate 

 murmur is heard when the ear is placed against the front or 

 the side of the chest. This is the vesicular or pulmonary 

 murmur. It is heard during inspiration, and is due to the 

 passage of air out of the smallest bronchi into the more spacious 

 infundibula in which they end. These two kinds of murmur 

 must be rigidly distinguished the laryngeal murmur, heard 

 in situations in which no lung-tissue intervenes between the 



