212 A MANUAL OF PHYSIOLOGY 



centre is further related to afferent nerves, of which the most 

 influential is the vagus, particularly its pulmonary fibres, 

 and its superior laryngeal branch. But almost any afferent 

 nerve may powerfully affect the centre ; and it is also 

 influenced by fibres passing 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 consequent death. The 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 (Hilton). 



Section of one phrenic is followed by paralysis of the 

 corresponding 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 severe 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 hemisection 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 contracting. But when, as usually happens, this is not 



