RESPIRATION. 131 



muscles, though capable of being modified to a certain extent by efforts of 

 the will, are of an automatic character, and called forth by nerve impulses 

 emanating from the medulla oblongata. The inspiratory center, generates 

 the nerve impulses, which, traveling outward through the phrenic and in- 

 tercostal nerves, excite contractions of the diaphragm and intercostal muscles, 

 respectively. This center is for the most part automatic in its action, though 

 it is capable of being modified by impulses reflected to it through various 

 sensor nerves. 



This center may be stimulated: 



1. Directly, by the condition of the blood. An increase of carbonic acid 

 or a diminution of oxygen in the blood causes an acceleration of the re- 

 spiratory movements; the reverse of these conditions causes a diminution 

 of the respiratory movements. 



2. Indirectly, by reflex action. The medulla may be excited to action 

 through the pneumogastric nerve, by the presence of carbonic acid in the 

 lungs irritating its terminal filaments; through the fifth nerve, by irritation 

 of the terminal branches; and through the nerves of general sensibility. 

 In either case this center reflects motor impulses to the respiratory muscles 

 through the phrenic, intercostal, inferior laryngeal, and other nerves. 

 Types of Respiration. The abdominal type is most marked in young 



children, irrespective of sex, the respiratory movements being effected by 

 the diaphragm and abdominal muscles. 



In the superior costal type, exhibited by the adult female, the respiratory 

 movements are more marked hi the upper part of the chest, from the first 

 to the seventh ribs, permitting the uterus to ascend in the abdomen during 

 pregnancy without interfering with respiration. 



In the inferior costal type, manifested by the male, the movements are 

 largely produced by the muscles of the lower portions of the chest, from the 

 seventh rib downward, assisted by the diaphragm. 



The respiratory movements vary according to age, sleep, and exercise, 

 being most frequent in early life, but averaging twenty a minute in adult 

 life. They are diminished by sleep and increased by exercise. There are 

 about four pulsations of the heart to each respiratory act. 



During both inspiration and expiration two sounds are produced: the one, 

 heard in the thorax, in the trachea, and larger bronchial tubes, is tubular in 

 character; the other, heard in the substance of the lungs, is -vesicular in 

 character. 



Amount of Air Exchanged in Respiration, and Capacity of Lungs. 



The tidal or breathing volume of air, that which passes in and out of the 

 lungs at each inspiration and expiration, is estimated at from twenty to 

 thirty cubic inches. 



