RESPIRATION 223 



I mm. as the negative inspiratory pressure in dogs). But since 

 the respiratory passages are abruptly narrowed at the glottis, 

 the variations of pressure must be greater below than above 

 it, and in general they must increase with the distance from 

 that orifice, being greater, for instance, in the alveoli than in 

 the bronchi. 



Relation of Respiration to the Nervous System. Unlike 

 the beat of the heart, the respiratory movements are entirely 

 dependent on the central nervous system. The ' centre ' 

 which presides over them is situated in the spinal bulb. It is a 

 bilateral centre that is, it has two functionally symmetrical 

 halves, one on each side of the middle line. Each of these 

 halves has to do more particularly with the respiratory muscles 

 of its own side, for destruction of one-half of the spinal bulb 

 causes paralysis of respiration only on that side. Anatomically 

 the respiratory centre has not been sharply localized ; but it 

 lies lower than the vaso- motor centre, not far from the point 

 of the calamus scriptorius. Stimulation of this region during 

 apncea (p. 231) is stated to cause co-ordinated inspiratory move- 

 ments and widening of the opening of the glottis through abduc- 

 tion of the vocal cords. The centre is brought into relation with 

 the muscles of respiration by efferent nerves. The phrenic nerves 

 to the diaphragm, and the intercostal nerves to the muscles 

 which elevate the ribs, are the most important of those con- 

 cerned in ordinary breathing. The respiratory 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 con- 

 sequent 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 con- 

 traction 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. 





