RESPIRATION 16S 



part in the process. Of course, if through injury the thorax is 

 punctured, then there is equal atmospheric pressure on both sides 

 of the walls of the lungs, and they collapse. (Fig. 119A.) 



Since the rhythmical respiratory movements are due to the 

 contractions of muscles, it follows that stimuli reach the muscles 

 from the nervous system; because muscles, excepting those of the 

 heart and alimentary canal, do not contract automatically. Now 

 we know that nerve impulses arise in the so-called respiratory 

 center in the lower part (medulla) of the brain, just before it 

 merges into the spinal cord, and pass by nerves to the muscles 

 involved. Usually it is the oxygen-carbon dioxide content of the 

 blood reaching the respiratory center which determines the rate 

 of the respiratory movements that it induces; though there are 

 nerves that carry nerve impulses from the lungs which contribute 

 to the rhythm of breathing by holding in check the activities of 

 the center itself. Indeed, the center can be influenced by stimuli 



Olfact ory c hamber External 

 X^^N. \ nares 

 Esophagus /§r \\ \ 1 



Internal 

 nares 



Tongue 



"* Glottis 

 Lung 



Fig. 119B. — Diagram to illustrate the respiratory movements of the Frog. 

 At left, the external nares are open and air enters the buccal cavity. At right, 

 the external nares are closed and the floor of the buccal cavity raised, forcing 

 air into the lungs. Note that absence of diaphragm and air-tight thoracic 

 cavity necessitates an entirely different mechanism from that in Mammals. 

 (From Hegner.) 



from nearly any part of the body: witness the effect of a cold 

 plunge on breathing. The action of the center is, of course, chiefly 

 involuntary since we breathe when we are asleep ; but obviously it 

 can be controlled to a considerable extent by the will because we 

 can ' hold the breath ' for some time by sending impulses to the 

 respiratory center which inhibit its discharge. If this center is 

 destroyed or the nerves are severed, respiratory movements imme- 

 diately and permanently cease. 



