146 HUMAN PHYSIOLOGY. 



The respiratory center, the so-called vital point, generates the 

 nerve impulses, which, traveling outward through the phrenic and 

 intercostal nerves, excite contractions of the diaphragm and inter- 

 costal muscles, respectively. This center is for the most part auto- 

 matic in its action, though it is capable of being modified by im- 

 pulses 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 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 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 in 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 move- 

 ments 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 exer- 

 cise, 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, 



