1. Compressed air above 5 atm. lessens the CO2 output, and lowers the body temperature in mice, rats, and young rabbits.

2. Oxygen at and above 1 atm. has the same effect. It is a sign of oxygen poisoning.

3. Compressed air at 10 atm. is more damaging–at least to small animals—than oxygen at 2 atm.

4. Compressed air increases the loss of body heat both because it is a better conductor, and because it is saturated with moisture.

5. The saturation of the air with moisture in caissons does not prevent evaporation from the body because the skin temperature is above that of the air. The wet air by damping fur or clothes increases loss of heat.

6. Highly compressed air may possibly interfere with the diffusion of CO2 from the alveolar air, and may, owing to increased friction, hinder the passage of air in and out of the air-tubes.

7. The nitrogen output in dogs is not altered in any noteworthy degree by exposure for six hours to 8 atm. air.

8. Inflammation and consolidation of the lungs is produced by exposure to 8 atm. air for over 24 hours. 1½ atm. of pure oxygen has a similar effect. The higher the oxygen tension the more rapidly does the inflammation ensure, e.g. 6 atm. O2 produces marked congestion in 2 hours.

9. It does not seem likely that inflammation of the lungs should be produced in the pressures and times of exposure usual in caissons.

10. Excised frog's hearts, muscles, and nerves are not rapidly poisoned by even 50 atm. O2. A heart will beat more than an hour exposed to this pressure. The vagus nerve endings appear to be paralysed by such exposure, while inhibition can be obtained by stimulating the crescent. The thin sartorius muscle is much more easily affected than the gastrocnemius, and soon gives a curve like a fatigue curve.

11. All animals investigated, vertebrates and invertebrates, are instantly convulsed and killed by exposure to 50 atm. O2.

12. Convulsions are frequently produced in vertebrates by exposure to 4—5 atm. O2, while exposure to 6—25 atm. O2 produces dyspnoea and coma, and the convulsive stage does not usually appear. Cleaning movements, salivation, gaping, jerky deep respiration, are symptoms which precede the convulsions, and coma soon follows them.

13. We have not observed convulsions with air pressures up to 12 atm. Salivation, dyspnoea, and coma are the symptoms.

14. The blood-gases increase in compressed air or oxygen according to Dalton's law, but the process of complete saturation of blood and tissues takes some time.

15. The circulation is unaffected mechanically by compressed air.

16. The cause of caisson-sickness is the escape of gas bubbles in the blood vessels and tissue fluids on decompression. An animal exposed for 4 hours to 8 atm. air and quickly decompressed is like an opened bottle of soda-water. The fluids of the body generally effervesce.

17. The effervescence can be studied in the circulation of the frog's web or bat's wing, the animals being enclosed in a suitable chamber. It takes a little time for the bubbles to grow to an appreciable size.

18. Recompression causes the bubbles to go into solution, and if applied quickly enough the circulation recommences.

19. The bubbles after rapid decompression can be seen post-mortem in the blood vessels, in the heart, retinae, aqueous humour, connective tissue spaces, etc. The alimentary canal is blown out with gas. The bubbles produce cyst-like cavities in solid organs, e.g. in the central nervous system, the liver. The cells are compressed round these cysts.

20. In the case of oxygen an animal may recover after an extraordinary amount of this gas has been set free by rapid decompression. The nerve cells are not killed by the oxygen bubbles, and the animals are convulsed and exhibit hyper-reflex-excitability.

21. The varying symptoms of caisson-sickness are due to the varying seat of the air emboli.

22. Young men escape caisson-sickness owing to the elasticity of their tissues, and greater facility for collateral pathways of circulation.

23. Animals can be safely exposed to 8 atm. of air for 4 hours if 2 hours be spent in gradual decompression. Such exposure can be safely repeated three times a week.

24. By the choice of suitable men, and proper regulation of the period of compression and decompression, caisson and diver's sickness can be avoided.