RESPIRATORY NERVE-CENTRES 603 



one side, the respiratory movements on that side become slower and are 

 no longer synchronous with the movements on the opposite side. This 

 shows that while the respiratory centres on the two sides normally act 

 together, being connected with each other by commissural fibres, each 

 one has independent connections with the pneumogastric on the corre- 

 sponding side of the body. 



In ordinary tranquil respiration the act of inspiration is reflex and 

 is due to an impression conveyed to the bulb through the pulmonary 

 branches of the pneumogastrics. It has lately been asserted that the 

 impression which gives rise to inspiratory movements is due to partial 

 collapse of the air-vesicles, and that when the vesicles are distended, an 

 impression is received by the bulb that gives rise to movements of expi- 

 ration. It is difficult, however, to reconcile this theory with the facts. 

 It does not account for the absence of respiratory movements in the 

 foetus, when the air-cells are entirely collapsed, or for certain phenomena 

 that follow inflation of the lungs with irrespirable gases. It is more 

 reasonable to assume that the impression, in tranquil respiration, is due 

 to the carbon dioxide contained in the air-cells, and that the movements 

 of expiration are almost entirely passive. 



In difficult respiration attended with a sense of suffocation and 

 violent inspiratory efforts, the sense is due to the circulation of non- 

 oxygenated blood in the respiratory centres. This was shown by a 

 number of experiments (Flint) begun in 1861 and carried on up to 1880. 



Vital Point (so called). Since it has been definitely ascertained that 

 destruction of a restricted portion of the gray substance of the bulb 

 produces instantaneous and permanent arrest of the respiratory move- 

 ments, Flourens and others have called this centre the vital knot, de- 

 struction of which is immediately followed by death. With the existing 

 knowledge of the properties and uses of the different tissues and organs 

 of which the body is composed, it is almost unnecessary to present any 

 arguments to show the unphilosophical character of such a proposition. 

 One can hardly imagine such a thing as instantaneous death of the 

 entire organism ; and still less can it be assumed that any restricted 

 portion of the nervous system is the one essential vital point. Probably, 

 a very powerful electric discharge passed through the entire cerebro- 

 spinal axis produces the nearest approach to instantaneous death ; but 

 even then it is by no means certain that some parts do not for a time 

 retain their physiological properties. In apparent death the nerves and 

 the heart may be shown to retain their characteristic properties ; the 

 muscles will contract under stimulus, and will appropriate oxygen and 

 give off carbon dioxide, or respire ; the glands may be made to secrete ; 

 and no one can assume that under these conditions the entire organism 



