RESPIRATION. 63 



Movements of the Glottis. At each inspiration the rima-glottidis is 

 dilated by a separation of the vocal cords, produced by the contraction of 

 the crico-arytenoid muscles, so as to freely admit the passage of air into the 

 lungs : in expiration they fall passively, together, but do not interfere with 

 the exit of air from the chest. 



Nervous Mechanism of Respiration. The movements of Respira- 

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

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

 nervous impulses emanating from the medulla oblongata. The Respiratory 

 centre, the so-called vital point, generates the nerve impulses, which, travel- 

 ing outward through the phrenic and intercostal nerves, excite contractions 

 of the diaphragm and intercostal muscles respectively. This centre is for 

 the most part automatic in its action, though it is capable of being modified 

 by impulses reflected to it through various sensory nerves. 



This centre 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 respi- 

 ratory 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 centre reflects motor impulses to the respiratory muscles 

 through the 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 yth 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 arid 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. 



