294 HUMAN PHYSIOLOGY 



passing into the trachea must be obvious (Fig. 137, d 

 and d'). 



Stimulation of the Cutaneous Nerves stimulates the in- 

 spiratory centre and causes a deep inspiration. This is seen 

 when cold water is dashed upon the skin, and is more clearly 

 demonstrated in animals with the vagi and upper nerve 

 tracts cut across. In such animals if the skin be touched an 

 inspiratory movement is made. 



The action of the respiratory centre is thus regulated by 

 many afferent nerves. Its activity is also modified by the 

 condition of the blood and lymph going to it. 



If the supply of blood to the medulla be interfered with 

 e.g. by ligaturing the arteries to the head breathing becomes 

 deeper and more laboured. The activity of the respiratory 

 centre is increased in the same way if the proper exchange 

 of gases between the blood and the air in the lungs is inter- 

 fered with. This may be due either to the withdrawal of 

 something necessary for the nutrition of the nerve cells of 

 the centre, or to the accumulation of products of activity. 



One of the most important substances yielded by the 

 blood for the nutrition of cells is oxygen. That the absence 

 of oxygen is alone sufficient to stimulate the activity of the 

 respiratory centre is shown by the fact that it' an animal is 

 made to breathe an atmosphere of nitrogen which prevents 

 fHel)Iood from obtaining oxygen, but docs not interfere with 

 its getting rid of its carbon dioxide, increase in the activity of 

 the respiratory centres is produced. 



This experiment does not, however, exclude the possibility 

 that the accumulation of carbon dioxide may also stimulate 

 the centre. If an animal be supplied with air in which there 

 is an abundance of oxygen, but which is loaded with carbonic 

 acid, the result is that the carbonic acid of the blood is not 

 got rid of. In such an animal the breathing becomes quicker, 

 deeper and more laboured ; but ultimately there is a tendency 

 for the breathing to diminish and for the animal to sink into 

 a state of deep coma. Since, under ordinary conditions, the 

 amount of oxygen in the blood does not materially vary, 

 while the amount of C0 2 may be considerably altered, it is 

 probable that the increase of C0 2 under certain conditions is 

 the more frequent stimulus to the respiratory centre. 



