REGULATION OF THE RESPIRATORY MOVEMENTS 1127 



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

 abdominal muscles in respiration ceases permanently, although respiration 

 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 con- 

 clude 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 scriptorius, which they designated nceud 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 

 circumscribed. 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 diaphragm on the same side. 

 There must however be commissural fibres joining the motor nuclei on the 

 two sides. If the right phrenic nerve be divided, after hemisection on the 

 left side, the left half of the diaphragm at once commences to contract 

 rhythmically with each respiration (Porter). It is evident that the cessation 

 of respiration after section of the cord is not due to a condition of shock of 

 the lower spinal centres, since it is possible for impulses to pass down the 

 cord and to cross over to the contra-lateral diaphragm nucleus immediately 

 after hemisection of the cord on the side of the nucleus. 



THE QUESTION OF SPINAL RESPIRATORY CENTRES. Several physiologists 

 e. g. Brown-S^quard, Langendorff, and Wertheimer, have described respiratory centres 



