258 A MANUAL OF PHYSIOLOGY 



many hours to a pressure of between i and 2 atmospheres of pure 

 oxygen causes pneumonia. He confirms Bert's observations on the 

 acute toxic effects produced by higher pressures, and supposes that 

 in the production of caisson disease the special action of the 

 oxygen at high pressure may play a part as well as the rapid de- 

 compression. 



When the air-pressure is diminished below a certain limit, 

 death takes place from asphyxia, more or less gradual 

 according to the rate at which the pressure is reduced. The 

 haemoglobin cannot get or retain enough oxygen to enable 

 it to perform its respiratory function; its dissociation tension 

 is no longer balanced by an equal or greater partial pressure 

 of oxygen in the air. The quantity of carbon dioxide in the 

 blood is also lessened. These belong to the chemical effects 

 of changes of pressure in the air of respiration. 



To such changes, as well as to the cold, some of the deaths in 

 high balloon ascents must be attributed. Messrs. Glaisher and 

 Goxwell supposed that they reached the height of 37,000 feet ; the 

 former became unconscious at 29,000 feet (8,800 metres), at which 

 height the amount of oxygen in the arterial blood would probably 

 not exceed TO volumes per cent., but recovered during the descent. 

 The symptoms of the ' mountain sickness ' so familiar to Alpine 

 climbers are also mainly due to deficiency of oxygen in the 

 blood. But evidence has been brought forward that changes in the 

 mechanics as well as in the chemistry of respiration are concerned, 

 and that there is something not connected with the want of oxygen 

 which diminishes the capacity for muscular work. This 'something ' 

 is perhaps a peculiar excitation of the nervous system in the fierce 

 light of those high levels, which acts not only on the retina, but on 

 the skin, and may even affect the distribution of the blood (Zuntz 

 and Schumburg). 



Cutaneous Respiration. It has already been remarked that a frog 

 survives the loss of its lungs for some time, respiration going on 

 through the skin. Indeed, it has been calculated that in the intact 

 frog as much as three-quarters of the total gaseous exchange is 

 cutaneous. In mammals the structure of the skin is different, and 

 respiration can only go on through it to a very slight extent. The 

 amount of carbon dioxide excreted in man, although only about 

 4 grm. or 2 litres in twenty-four hours, is much greater than cor- 

 responds to the quantity of oxygen absorbed through the skin. It 

 has been asserted, and no doubt with justice, that some at least of 

 the carbon dioxide given off is due to putrefactive processes taking 

 place on the surface of the body. Such processes, as has already 

 been pointed out, seem also responsible in part for the heavy odour 

 of a ' close ' room. For no harmful products appear to be exhaled 

 from the skin when it is properly cleansed. In spite of the romantic 



