14 Messrs. L. Hill and M. Greenwood, Jun. — [Oct. 1, 
Table III. 
Ages. Actual number affected. Theoretical number. 
15—20 if 14°43 
20—25 35 38°04 
25—30 43 39 °88 
30—35 32 23 °87 
35—40 19 16 
40—45 8 9 ‘97 
| 45—50 5 0°79 
and xy? is 39:13, or the odds are about three in a million. It will be seen, 
therefore, that there is some evidence in favour of an age bias, but the 
statistics are not sufficiently detailed to give much information. 
Returning to (i), (11), and (11), it is clear that we must have considerable 
difficulty in isolating (1) and (i1) under experimental conditions. Thus, in 
dealing with such animals as rats, it is not easy to obtain a large number of 
the same age but markedly different body weights. If we attempt to overcome 
the difficulty by comparing animals of different species, a new factor is 
involved, the importance of which cannot at present be estimated. 
It must, therefore, be admitted that in the results about to be detailed the 
observed differences cannot be ascribed wholly to body mass; other changes 
associated with growth and decay may be influential, and we must certainly 
not compare directly animals of different species. With these reservations, 
regarding which more will be said later, we shall demonstrate an appreciable 
difference in liability to caisson disease when animais of different weights are 
employed. | 
It is known that the velocity of the circulation and rate of respiratory 
exchange in small mammalia is greater than in large animals, the relatively 
larger surface exposure in the former necessitating a higher rate of meta- 
bolism. Since the saturation of the tissues with gas, together with its 
removal from them, are functions of the blood, it follows that the processes 
should require less time in small than large animals. If, then, we expose 
large and small animals to the influence of compressed air for so long a time 
that both will contain large quantities of dissolved gas, a decompression rate 
dangerous for the former should be safe for the latter. 
Our experiments have been carried out on rats, mice, cats, rabbits, and 
guinea-pigs. 
(a) Lats. 
In all experiments the animals were exposed to a pressure of +105 lbs. 
for periods varying from a-half to two hours; in most cases the exposure was - 
