186 RESPIRATION. 



as secondary muscles of respiration ; and the nerves supplying 

 them, including especially the facial, pneumogastric, spinal 

 accessory, and external respiratory nerves, were classed by Sir 

 Charles Bell with the phrenic, as the respiratory 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 respira- 

 tory acts. That which is peculiar in the nervous influence, 

 directing the extraordinary movements of respiration, is, that 

 so many nerves are combined toward 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 de- 

 stroyed, no movement adapted to respiration can be performed 

 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 movement of the 

 diaphragm when their connection with the medulla oblongata 

 is cut off, though their connection with the spinal cord may be 

 uninjured. 1 



Effects of the Suspension and Arrest of Respiration. 



These deserve some consideration, because of the illustra- 

 tion which they afford of the nature of the normal processes 

 of respiration and circulation. When the process of respira- 

 tion is stopped, either by arresting the respiratory movements, 

 or permitting them to continue in an atmosphere 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 re- 

 turn 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 condition, then, in which a suffocated, or asphyxiated 

 animal dies is, commonly, that the left side of the heart is 

 nearly empty, while the lungs, right side of the heart, and 

 other organs, are gorged with venous blood. To this condi- 

 tion many things contribute. 1st. The obstructed passage of 



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

 and more particularly considered in the section treating of the me- 

 dulla oblongata and pneumogastric nerves. 



