238 RESPIRATION. 



system of nerves. There appears, however, no propriety in 

 making a separate system of these nerves, since their mode 

 of action is not peculiar, and many besides them co- 

 operate in the respiratory acts. That which is peculiar in 

 the nervous influence, directing the extraordinary move- 

 ments of respiration, is, that so many nerves are com- 

 bined towards one purpose by the power of a distinct 

 nervous centre, the medulla oblongata. In other than 

 respiratory movements, these nerves may act singly or 

 together, without the medulla oblongata ; but after it is 

 destroyed, no movement adapted to respiration can be per- 

 formed by any of the muscles, even though the part of the 

 spinal cord from which they arise be perfect. The phrenic 

 nerves, for example, are unable to excite respiratory move- 

 ments of the diaphragm when their connection with the 

 medulla oblongata is cut off, though their connection with 

 the spinal cord may be uninjured.* 



Effects of the Suspension and Arrest of Respiration. 

 These deserve some consideration, because of the illus- 

 tration which they afford of the nature of the normal 

 processes of respiration and circulation. When the process 

 of respiration is stopped, either by arresting the respiratory 

 movements, or permitting them to continue in an atmo- 

 sphere deprived of uncombined oxygen, the circulation of 

 blood through the lungs is retarded, and at length stopped. 

 The immediate effect of such retarded circulation is an 

 obstruction to the exit of blood from the right ventricle : 

 this is followed by delay in the return of venous blood to 

 the heart ; and to this succeeds venous congestion of the 

 nervous centres and all the other organs of the body. In 

 such retardation, also, an unusually small supply of blood 

 is transmitted through the lungs to the left side of the 

 heart ; and this small quantity is venous. 



The influence of the nervous system in respiration will be again and 

 more particularly considered in the section treating of the medulla ob- 

 longata and pneumogastric nerves. 



