440 THE BRAIN. 



occupying the lateral and posterior portions of the medulla, at the 

 point of origin of the pneumogastric nerves. This portion of the 

 brain has long been known to be particularly essential to the pre- 

 servation of life ; so that it has received the name of the " vital 

 point," or the " vital knot." All the other parts of the brain may 

 be injured or removed, as we have already seen, without the imme- 

 diate and necessary destruction of life ; but so soon as the medulla 

 oblongata is broken up, and its ganglion destroyed, respiration 

 ceases instantaneously, and the circulation also soon comes to an 

 end. Eemoval of the medulla oblongata produces, therefore, as its 

 immediate and direct result, a stoppage of respiration : and death 

 takes place principally as a consequence of this fact. 



Flourens and Longet have determined, with considerable accu- 

 racy, the precise limits of this vital spot in the medulla oblongata. 

 Flourens ascertained that in rabbits it extended from just above 

 the origin of the pneumogastric nerve, to a level situated three lines 

 and a half below this origin. In larger animals, its extent is pro- 

 portionally increased. Longet ascertained, furthermore, that the 

 properties of the medulla were not the same throughout its entire 

 thickness ; but that its posterior and anterior parts might be de- 

 stroyed with comparative impunity, the peculiarly vital spot being 

 confined to the intermediate portions. This vital point accordingly 

 is situated in the layer of gray matter, imbedded in the thickness 

 of the restiform bodies, which has been previously spoken of as 

 giving origin to the pneumogastric nerves. 



The precise nature of the connection between this ganglion and 

 the function of respiration may be described as follows. The 

 movements of respiration, which follow each other with incessant 

 regularity through the whole period of life, are not voluntary 

 movements. We may to a certain extent, hasten or retard them 

 at will, but our power over them, even in this respect, is extremely 

 limited ; and in point of fact they are performed, during the greater 

 part of the time, in a perfectly quiet and regular manner, without 

 our volition and even without our consciousness. They continue 

 uninterruptedly through the deepest slumber, and even in a con- 

 dition of insensibility from accident or disease. 



These movements are the result of a reflex action taking place 

 through the medulla oblongata. The impression which gives rise 

 to them originates principally in the lungs, from the accumulation 

 of carbonic acid in the pulmonary vessels and air-cells, is trans- 

 mitted by the pneumogastric nerves to the medulla, and is thence 



