234 INFLUENCE OF ATMOSPHERIC PRESSURE 



cocks, whereby the men can be subjected to compression and 

 decompression on entering or leaving the caisson. 



Ten metres of water correspond to one atmosphere. Thus for 

 every 10 m. or 33 ft. an air pressure of +15 Ibs. to the sq. inch 

 or 1 atm. is required to keep out the water. In the case of a 

 diver, the conditions are the same : compressed air is delivered 

 through a valved inlet tube to his helmet ; the air escapes through 

 a valved outlet into the water ; the helmet is joined to the dress 

 in air-tight fashion, and the water is kept out of the dress and 

 helmet by the compressed air. The pressure of the air must 

 always be just in excess of that of the water. At 100 ft. a diver 

 is exposed to 4 atm. and at 200 ft. to 7 atm. pressure. It was 

 soon found that deep divers and men working under high pres- 

 sure in caissons are subject to various symptoms- which occurred, 

 not during compression, but after decompression. The minor 

 symptoms are severe pains in the joints and muscles, popularly 

 called " bends " or " the pressure." The severe symptoms are 

 protean, and include epigastric pain and vomiting, respiratory 

 embarrassment, paraplegia, auditory vertigo, sudden loss of 

 consciousness, and death. In the caissons used to sink the piles 

 of the St. Louis bridge the pressure equalled 4J atm. The 

 number of workers was 600, of whom 14 died and 119 were more 

 or less affected. There were 53 cases of paralysis of the lower 

 limbs. It is recorded that the controllers of the air-lock, who 

 were subjected to compression and decompression every few 

 minutes, and visitors who stayed but a few minutes in the St. 

 Louis caisson, never suffered. Among divers, paraplegia is so 

 common a symptom that it is known as "divers' palsy." The 

 Greek sponge and coral divers have lost as many as a score of 

 men in a year. The men are stricken shortly after returning to 

 the boats. 



Many medical writers, knowing the effects of compression and 

 of cupping locally applied, have ascribed the causation of com- 

 pressed-air illness to the mechanical effects of the pressure. They 

 have supposed that the blood is expressed from the outer part of 

 the body and congested within, and that the illness arises from 

 this congestion. They have, in particular, supposed that the 

 central nervous system, enclosed in a bony case as it is, is pro- 

 tected from the pressure, and hence is especially liable to congestion. 

 Such suppositions neglect the fact that the compressed air is 



