RESPIRATION 143 



capable of considerable adjustment ; they hold more blood during 

 inspiration than expiration, and in this way may be regarded 

 as a safety-valve to the heart. The important practical ques- 

 tions of work, ' condition,' and fatigue will be again referred to 

 in the chapter dealing with the Muscular System. 



Air vitiated by Respiration was at one time believed to be 

 poisonous, either on account of its deficiency in oxygen, its 

 increase in carbon dioxide, or to the organic matter mixed up 

 with it. It is now generally admitted that the ill-effects of vitiated 

 air are mainly due to the stagnation of the air and the warm 

 and humid atmosphere, by which the respiratory exchange and 

 body metabolism are affected. Even the number of bacteria 

 in the air is no guide to purity ; there may be fewer in expired air 

 than in the same air before inspiration, in consequence of their 

 being arrested in the lungs. Nevertheless, modern inquiry 

 supports the principle contained in the old view, of the evil 

 resulting from breathing atmosphere charged with C0 2 . When 

 the gas accumulates to the extent of 4 per cent., rapid breathing 

 and general distress begin to be evident. 



The amount of air required for effective ventilation is con- 

 veniently based on the amount of permissible impurity present, 

 as judged by the proportion of carbon dioxide existing. Many 

 observations show that when 002 per cent, of carbon dioxide 

 is present, in addition to that in the air as a normal impurity — 

 viz., 003 per cent. — the ventilation may be regarded as effective. 

 On this basis, if the rate of C0 2 production by any given animal 

 is known, it is easy to calculate the number of times the air 

 of the building should be changed, in order to maintain it in a 

 pure condition. 



Respiratory Murmur. — An accurate acquaintance with the 

 normal respiratory murmur is essential to the physician. The 

 air sounds both of inspiration and expiration should be heard 

 all over the chest, the inspiratory murmur being louder and 

 better marked than the expiratory ; in fact, in many perfectly 

 healthy chests the expiratory murmur can scarcely be heard. 

 The normal murmur, whether inspiratory or expiratory, is soft 

 in character ; there is no harshness. The sound is best repre- 

 sented by the noise made by the stream of air which issues from 

 a pair of hand-bellows when gently blown. 



The respiratory murmur, also known as the vesicular murmur, 

 is caused by the friction of the air entering the alveoli. In those 

 portions of the lung lying close to the bronchi and larger tubes 

 there is, in addition to the vesicular murmur, a sound produced 

 by the trachea and glottis. This is not distinct from the vesicular 

 sound, but is added to it, the result being that the respiratory 

 murmur over the tubes is louder than elsewhere. The expira- 



