RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 269 



widening of the opening of the glottis through abduction of the 

 vocal cords. The centre is brought into relation with the muscles 

 of respiration by efferent nerves.; (The phrenic nerves to the dia- 

 phragm, and the intercostal nerves to the muscles which elevate 

 the ribs, are the most important of those concerned in ordinary 

 breathing.^ The respiratory centre is further related to afferent 

 nerves, of which the most influential are those which supply the' 

 respiratory tract itself, particularly the pulmonary fibres and superior 

 laryngeal branch of the vagus. But almost any afferent nerve may 

 powerfully affect the centre ; and it is also influenced by fibres pass- 

 ing to it from the higher parts of the central nervous system. 



Section of the spinal cord in animals above the origin of the 

 phrenic nerves causes complete paralysis of respiration, and con- 

 sequent death. fThe phrenics arise from the third and fourth 

 cervical nerves, and are joined by a branch from the fifth; and in 

 man fracture of any of the four upper cervical vertebrae is as a rule 

 instantly fatal. But in one case respiration was carried on, and 

 life maintained for thirty minutes, merely by the contraction of the 

 muscles of the neck and shoulders in a man entirely paralyzed 

 below this level (Bell). Section of the cord just below the origin of 

 the phrenics leaves the diaphragm working, although the other 

 respiratory muscles are paralyzed. A case has been recorded of a 

 man in whom, from disease of the spine in the lower cervical region, 

 all the ribs became completely immovable. He was able to lead 

 an active life, and to carry on his business, although he breathed 

 entirely by his diaphragm and abdominal muscles. 



Section of one phrenic is followed by paralysis of the correspond- 

 ing half of the diaphragm, section of both phrenics by complete 

 paralysis of that muscle, and although respiration still goes on by 

 means of the muscles which act upon the ribs, it is usually inadequate 

 to the prolonged maintenance of life. In the horse, however, not only 

 has survival been seen after this operation, but the animal, after 

 the first temporary increase in the frequency of the breathing had 

 disappeared, could be driven in a light vehicle without any marked 

 dyspnoea. The phrenic nuclei in the two halves of the cord are 

 connected across the middle line. For when a semisection of the 

 cord is made between this level and the respiratory centre in the 

 medulla, respiratory impulses are still able to reach both phrenic 

 nerves. In some animals both halves of the diaphragm go on con- 

 tracting. But when, as usually happens, this is not the case, and 

 the diaphragm on the side of the semisection has ceased to act, it 

 at once begins to contract again when the opposite phrenic nerve 

 is cut, and the respiratory impulse, descending from the bulb, is 

 blocked out from the direct, and forced to follow the crossed path. 

 It has been shown that the crossing takes place at the level of the 

 phrenic nuclei, and nowhere else (Porter). 



