238 HUMAN PHYSIOLOGY 



of other centres of the central nervous system is not directly 

 fatal. The centres of the medulla oblongata have already 

 been mentioned in the chapter on metabolism and their 

 properties have there been described in detail, so that a 

 simple enumeration will suffice here. 



1. The respiratory centre (see page 83). By this 

 centre the muscles which cause alternate inspiration and 

 expiration (the diaphragm and the external intercostal for 

 inspiration, the internal intercostal for expiration) are stimu- 

 lated in an orderly manner. Its activity is dependent upon 

 the need of oxygen by the body, for lack of oxygen and 

 accumulation of carbon dioxide in the blood act as normal 

 stimuli for respiration. Reflexly the respiratory centre is 

 regulated by centripetal nerves, namely, the fibres of the 

 vagus leading from the lungs to the centre. The inspiratory 

 fibres of the vagus are stimulated during expiration, while the 

 expiratory fibres are stimulated during inspiration. 



2. The centres for the organs of circulation (see page 

 74). These are: 



(a) The cardio-inhibitory centre (of the inhibitory vagus 

 fibres). 



(&) The centre for the sympathetic nerve from the cervical 

 and the first thoracic ganglia, which carry the accelerating 

 fibres to the heart. 



(c) The centre for the constriction of blood vessels. 



(d) The centre for the dilation of the blood vessels. 

 These centres serve to regulate the pressure of the blood 



stream and its distribution in the various parts of the body 

 according to existing needs, by means of changes in the 

 number and strength of the heart-beats and in the tonus of 

 the muscles of the blood vessels. 



The cardio-inhibitory centre and the vaso-constrictor 

 centre are tonic. They are stimulated by lack of oxygen 

 and accumulation of carbon dioxide in the blood. It appears 

 that this stimulation serves to protect the heart from too 

 speedy exhaustion during asphyxia, by decreasing its 

 activity, and to compensate the resulting reduction in blood 



