564 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



When this region was destroyed he found that respiratory movements ceased 

 and death ensued, consequently he termed it the nceud vital, or vital knot. 

 The results of various investigations show, however, that Flourens' area, as 

 well as certain other parts of the medulla oblongata that have been looked 

 upon by others as being respiratory centres, are not such, but are largely or 

 wholly collections of nerve-fibres which arise chiefly in the roots of the vagal, 

 spinal accessory, glosso-pharyngeal, and trigeminal nerves, and which there- 

 fore are probably nerve-paths to and from the respiratory centre. Moreover, 

 excitation of the nceud vital does not excite respiratory movements, but simply 

 increases the touicity of the diaphragm ; nor is the destruction of the area 

 always followed by a cessation of respiration. While the precise location of 

 the centre is still in doubt, there is abundant evidence to justify the belief in 

 its existence in the lower portion of the spinal bulb. 



The centre is bilateral, one half being situated on each side of the median 

 line, the two parts being intimately connected by commissural fibres, thus con- 

 stituting physiologically a single centre. This union may be destroyed by 

 section along the median line. Each half acts more or less independently of, 

 although synchronously with, the other, and each is connected with the lungs 

 and the muscles of respiration of the corresponding side. These facts are 

 rendered manifest in the following observations : If a section be made in the 

 median line so as to cut the commissural fibres, the respiratory movements on 

 the two sides continue synchronously ; if now the portion of the centre on the 

 one side be destroyed, the respiratory movements on the corresponding side tem- 

 porarily or permanently cease. If after section in the median line one pneumo- 

 gastric nerve be divided, the sensory impulses conveyed from the lungs on the 

 side of section to the corresponding half of the respiratory centre are prevented 

 from reaching the centre, causing the movements of the respiratory muscles on 

 the same side to be slower and the inspirations stronger as compared with those 

 on the opposite side ; if both pneumogastrics be divided, and the central end of 

 one of the cut nerves be excited high in the neck by a strong current, the respi- 

 ratory movements on the same side may be arrested, yet they may continue on 

 the opposite side. These facts indicate that each half is in a measure inde- 

 pendent of the other. The operations in the two parts are, however, inti- 

 mately related, as shown by the fact that if the commissural fibres between 

 the halves are intact, excitation or depression of one half is to a certain degree 

 shared by the other. Thus, after section of one vagus not only are the respi- 

 ratory movements less frequent and the inspirations stronger on the side of 

 the section, but there is a corresponding condition on the opposite side ; simi- 

 larly, excitation of the central end of the cut nerve increases the respiratory 

 rate both on the same and on the opposite side. Consequently, while there is 

 more or less independence of the halves, the two are physiologically so 

 intimately associated as to constitute a common or single centre. 



Moreover, each of the halves may be supposed to consist of two distinct 

 portions, one of which, upon excitation, gives rise to contraction of inspiratory 

 muscles, the other to contraction of expiratory muscles ; hence they are spoken 



