58 HUMAN PHYSIOLOGY. 



1. Directly i by the condition of the blood. An increase of carbonic 

 acid or a diminution of oxygen in the blood causes an acceleration of the 

 respiratory 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 irrita- 

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

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

 through \\\e phrenic, intercostals, 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 in the upper part of the chest, from the 1st to 

 the 7th ribs, permitting the uterus to ascend in the abdomen during preg- 

 nancy without interfering with respiration. 



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

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

 7th rib downward, assisted by the diaphragm. 



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

 being most frequent in early life, but averaging 20 per 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 inspiration 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 20 to 30 

 cubic inches. 



The Complemented air is that amount which can be taken into the lungs 

 by a forced inspiration, in addition to the ordinary tidal volume, and 

 amounts to about no cubic inches. 



The Reserve air is that which usually remains in the chest after the 

 ordinary efforts of expiration, but which can be expelled by forcible expira- 

 tion. The volume of reserve air is abojit loo cubic inches. 



The Residual air is that portion which remains in the chest and cannot 



