THE RESPIRATORY SYSTEM. 283 



The symptoms caused by this obstruction, and known as caisson 

 disease, are very varied, and include paralysis, severe abdominal pain, 

 and collapse. The disease is prevented by allowing the man to pass 

 from the caisson into a special air chamber, in which the pressure is 

 gradually lowered to that of the atmosphere so as to prevent any 

 sudden evolution of nitrogen. 



CARBON MONOXIDE POISONING. 



The affinity of carbon monoxide for haemoglobin is about 130 

 times as great as that of oxygen ; and when air containing even a 

 small percentage of carbon monoxide is breathed, the oxyhsemoglobin 

 is replaced by qarbon monoxide haemoglobin, and asphyxia is produced. 

 The fatal effects of breathing air containing coal gas, in which carbon 

 monoxide is present, are brought about in this way ; but death may 

 often be prevented by the administration of pure oxygen, which not 

 only increases the amount of oxygen dissolved in the blood and carried 

 to the tissues, but, by its mass action, gradually displaces the carbon 

 monoxide from its combination with haemoglobin. 



SECTION VI. 



THE INFLUENCE OF THE RESPIRATORY MOVEMENTS 

 ON THE CIRCULATION. 



On examining a tracing of the blood pressure, it is often noticed 

 that the pressure shows oscillations corresponding with each respiratory 

 movement, rising a little with each inspiration and falling during 

 expiration. Further, the pulse is more frequent during inspiration 

 and less frequent during expiration (fig. 114). The difference in the 

 pulse rate is due to a slight diminution of the tone of the vagus during 

 inspiration, which allows the heart to beat more rapidly; it is abolished 

 by section of the vagi, but the respiratory oscillations of the blood 

 pressure are not affected by this procedure. 



At the end of expiration, the pressure inside the cavity of the 

 chest is slightly below atmospheric pressure and the pressure on the 

 walls of the great veins and of the heart is negative, whereas the 

 pressure in the jugular vein, for example, is slightly higher than that 

 of the atmosphere. Owing to this difference of pressure in the vessels 

 within and outside the chest, blood tends to be sucked along the great 

 veins and into the heart. With each inspiration the negative pressure 

 is increased, and the flow of blood into the heart becomes more rapid. 

 The thin-walled auricles are also slightly dilated- by the negative 



