158 ELEiMENTARY PHYSIOLOGY less 



nervous centre lodged in the spinal bulb, which has been 

 called the respiratory centre. 



This centre is situated (see Fig. 49, R.G.) close to the 

 two previously described (Figs. 22 and 23) as the vaso- 

 motor and cardio-inhibitoi-y centres (pp. 69 and 75). Im- 

 pulses arise in this centre, pass down the spinal cord, 

 and leaving the cord along certain nerves, reach the 

 various muscles by whose contractions the movements of 

 respiration are produced. The respiratory muscles contract 

 only when they receive these impulses, and therefore all 

 the movements of respiration depend upon the activity 

 of this centre, and cease at once on injury of this j)art of 

 the spinal bulb. 



The action of the centre is primarily antomatic ; in 

 other words the impulses it sends out appear to be the 

 result of changes started irithin itself, in the same way 

 that the beat of the heart is automatic as the outcome of 

 changes started in the muscle-tissue of which it is made 

 up. This primary automatism of the respiratory centre 

 is sul)ject, however, to control by impulses reaching it 

 from outlying parts of the body, and more particularly 

 by changes in the condition or quality of the blood which 

 circulates in the capillaries of the centre itself, iji a 

 way to be described presently. 



The intercostal muscles are supplied by intercostal 

 nerves coming from the spinal cord in the region of the 

 back (Fig. 49, ION, ICN, ICN), and the muscular filjres of 

 the diaphragm are supplied by two nerves, cme on each side, 

 called the phrenic nerves (Fig. 49, Phr.), which starting 

 from certain of the spinal nerves in the neck, dij) into the 

 thorax at the root of the neck, and find their way through 

 the thorax by the side of the lungs to the diaphragm, over 

 which they are distributed. Now from the nervous 

 respiratory centre in the spinal bulb impulses at repeated 

 intervals descend along the ujiper part of the spinal cord 

 and, passing out by the phrenic and intercostal nerves 



