KESPIRATION 175 



appointing, because the system is suffering much less from 

 deficiency of oxygen than from accumulation of carbonic acid. 

 Substituting oxygen for air does not facilitate the escape of 

 carbonic acid. 



The nervous mechanism of respiration has been the subject 

 of much investigation and of many experiments, without, it 

 must be confessed, the development of a quite complete or 

 satisfactory theory. Rospiration is a rhythmic process. 

 About seventeen times in a minute the intercostal and dia- 

 phragmatic muscles contract. Inspiration is immediately 

 followed by expiration, the falling movement being due, 

 as already explained, to the elasticity of ' the lungs, which 

 are stretched during inspiration. A slight pause intervenes 

 between the end of expiration and the commencement of the 

 next inspiratory movement. Tranquil respiration is a succes- 

 sion of reflex inspiratory movements, the depth of which varies 

 according to the needs of the body that is to say, according 

 to the condition of the blood. If the need for aeration of the 

 blood becomes urgent, the depth of inspiration is increased, and 

 expiration also becomes an active movement, certain muscles, 

 especially those of the abdomen, being called into play. In 

 this condition two sets of reflex actions alternate. A large 

 number of nerves are concerned even in tranquil respiration. 

 If a man in falling " breaks his back " at the junction of the 

 cervical and thoracic regions, costal respiration ceases. The 

 series of intercostal nerves which arises from the dorsal spinal 

 cord below the level at which it is injured are thrown out of 

 action. Diaphragmatic respiration still continues, because the 

 nerve of the diaphragm, the phrenic, arises from cervical roots. 

 The lungs are supplied by the vagus nerve. This nerve joins 

 the medulla oblongata as one of a group of three glosso- 

 pharyngeal, vagus, and spinal accessory which by a large 

 number of roots enter the groove between the olive and the 

 restif orm body. The vagus is the channel along which afferent 

 impulses from the lungs enter the medulla. Such impulses 

 call for respiratory movements. Cutting both vagi, however, 

 does not put an end to respiration. Inspiratory movements 

 continue, but they are much deeper and separated by much 

 longer pauses. Such a form of respiration is inefficient. The 

 blood is not properly aerated. The animal fells into a condi- 



