REGULATION OF RESPIRATORY MOVEMENTS 1201 



endowed with a special sensibilty to changes in the composition of 

 the blood circulating through its vessels. Experiment shows that the 

 latter method is employed in the organism for the regulation of 

 the respiratory movements. If the spinal cord be cut across, below 

 the seventh cervical nerve-roots, the action of the intercostal and 

 abdominal muscles in respiration ceases permanently, although respira- 

 tion is still continued by the rhythmic activity of the diaphragm and 

 the other muscles supplied by nerves leaving the central nervous 

 system above the point of section. Division of the cord at the first 

 or second cervical nerve abolishes the action of the diaphragm, though 

 the movements of the muscles supplied by the facial, vagus, and spinal 

 accessory nerves continue. A section of the brain-stem through 

 the mid-brain leaves the respiratory movements unaltered, and the 

 same absence of effect as concerns these movements may often be 

 obtained when a section is carried across the upper part of the medulla 

 about the level of the strice acousticce. We must conclude from these 

 experiments that the motor nuclei of the cord are subject to and 

 normally thrown into activity by impulses originating in the medulla 

 oblongata and transmitted therefrom down the spinal cord. 



Many experiments have been made with the idea of locating the 

 position of the medullary respiratory centre more accurately. The 

 first experiments on this point were made at the beginning of last 

 century by Legallois, whose observations were confirmed and extended 

 by Flourens. These observers described the respiratory centre as 

 limited to a small area at the level of the apex of the calamus scripto- 

 rius, which they designated noeud vital on account of the fact that 

 destruction of this area was at once fatal by paralysis of respiration. 

 Later experiments have shown that the centre is not quite so circum- 

 scribed. In the first place, it is bilateral, each centre presiding more 

 especially over the muscles of the same side of the body, so that 

 longitudinal section in the middle line does not destroy the respiratory 

 movements. Other observers have located the centre in the situation 

 of the solitary bundle (' respiratory bundle of Gierke '), which is made 

 up of the descending branches of the vagus nerve after they have 

 entered the medulla, while, according to Gad, the respiratory centre 

 is diffused over a considerable area of the formatio reticularis on either 

 side of the medulla. There is no doubt that this centre is in close 

 connection with the central terminations of the vagus nerves. 



From the centre on each side the efferent impulses to the motor 

 nuclei of the respiratory muscles pass down in the deeper portions 

 of the lateral columns of the cord. Hemisection of the cervical cord, 

 e.g. on the right side, causes cessation of the contractions of the dia- 

 phragm on the same side. There must, however, be commissural 

 fibres joining the motor nuclei on the two sides. If the right phrenic 



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