NERVOUS MECHANISM OF RESPIRATION. 287 



which the nerve fibres to the respiratory muscles arise can, like the 

 motor cells of other parts of the cord, be affected by impulses reaching 

 them along various paths. We may conclude that their normal 

 activity in respiration is determined by impulses reaching them from the 

 medulla oblongata. But they can also be affected along other tracts 

 derived ultimately from the posterior roots, either at the same or at a 

 higher or lower level of the cord. We know that the spinal cord, 

 separated from the upper parts of the central nervous system, under the 

 influence of asphyxia or strychnine, gives rise to spasmodic motor dis- 

 charges, which have a tendency to take on a more or less rhythmic 

 character. These rhythmic discharges affect the limbs as well as the 

 muscles of the trunk, and it is evident that, if we were tracing the 

 movements of the thorax or diaphragm, they might resemble the effect 

 produced by normal respiratory movements. But, strictly speaking, we 

 cannot locate respiratory centres in the cord, since this property of 

 rhythmic discharge, in consequence of asphyxia or strychnine ,is shared 

 by all the motor cells or centres of the cord, and is not limited to those 

 supplying the respiratory muscles. As a matter of fact the pseudo- 

 respiratory movements observed under these conditions are extremely 

 inadequate for the renewal of air in the lungs, and one often obtains 

 simultaneous spasms of inspiratory and expiratory muscles. So long, 

 therefore, as an animal is warm-blooded, they are of very little use for 

 the maintenance of life. In Wertheimer's experiments, in consequence 

 of the prolonged ventilation of the lungs, the animal had become practic- 

 ally cold-blooded, and under these circumstances it is not wonderful 

 that it should have survived cessation of artificial respiration for forty- 

 five minutes. In an anaesthetized and curarised animal, after a long 

 experiment in which the animal has had time to get cold, the heart 

 may go on beating for half an hour or more after artificial respiration 

 has been discontinued. 



Moreover, Porter 1 has shown that such a lesion as dividing the 

 medulla oblongata below the respiratory centre does not produce an 

 appreciable amount of shock, i.e., does not affect injuriously the subor- 

 dinated activities of these spinal respiratory centres. Thus, if the bulb 

 be hemisected below the respiratory centre, the respiratory movements 

 of the diaphragm at once cease on that side. This result, according to 

 Brown-Sequard or Langendorff, would be due to shock or inhibition of 

 the diaphragm centre in the cord. If now the phrenic nerve be divided 

 on the other side, the diaphragm on the side of the hemisection at once 

 commences to contract, the impulses coming from the other side of the 

 medulla oblongata having crossed from one side of the spinal cord to the 

 other by means of commissural fibres situated at the level of origin of 

 the phrenic nerves. This immediate result shows that the diaphrag- 

 matic centre in the cord was previously inactive, not in consequence of 

 inhibition due to the hemisection, but because the impulses from the 

 respiratory centre in the medulla oblongata on the same side had been 

 cut off by the hemisection. 



We may conclude, therefore, that for the normal performance and 

 co-ordination of the respiratory movements, the integrity of certain 

 parts of the medulla oblongata situated on each side of the median line 

 is necessary, and we are therefore justified in giving to these parts the 

 name of respiratory centre. 



1 Journ. Physiol., Cambridge and London, 1895, vol. xvii. p. 455. 



