206 
Proceedings of the Royal Irish Academy. 
of tlie calibre of the vessels, and increased obstacle to the propulsive 
force of the heart. 
The vessels were seen to contract on the conjunctivce, in the retina, 
and in the ear of the rabbit. In regard to respiration it produces a 
twofold effect — augmentation of the capacity of the lungs through 
dilatation of the vesicular structure, and the introduction of a larger 
quantity of air ; the latter effect is in part due to expansion of the air- 
sacs, but mainly to the condensed character of the air inspired. 
From the observations hitherto made it may be concluded that the 
effects upon the human body of protracted immersion in, and respira- 
tion of, compressed air are : — 
1. Slowing of the respiration, and derangement of its normal 
rhythm. 
2. Eetardation of the pulse. 
3. Contraction of the superficial blood vessels. 
4. Dilatation of the air-sacs and vesicles of the lungs. 
5. Pain in the ears, and loss of hearing. 
6. jN"asal voice, and inability to whistle. 
7. Muscular paralysis. 
8. Depression of temperature. 
9. Copious perspiration. 
10. Diminished exhalation of carbonic acid. 
It will be noticed that most of these effects are in direct contrast 
with those previously mentioned as arising from exposure to, and in- 
halation of, highly rarified air, and taken collectively they constitute 
a complex and very difficult problem in physiology. Without under- 
taking in this Paper to discuss them in a complete manner, I shall 
notice a few of the more interesting, and offer such remarks thereon 
as the time and space now at my disposal admit. 
Oppression, slow pulse and breathing, and irregularity of the lat- 
ter, seem due in great measure to repulsion of blood from the surface 
upon the internal organs by the mechanical pressure of the condensed 
air, and consequent congestion of the respiratory and circulatory 
nerve-centres. Hence, irritation of the roots of the pneumogastric 
nerves, and the usual effects of such, namely, slow and irregular 
breathing, and inhibition of the heart's action. In regard to the 
breathing there is, however, another cause in operation which must not 
be omitted, namely, the alteration in the relative diffusibility of the 
gases within, and those external to the body, and the consequent mo- 
dification of Graham's law in a sense unfavourable to respiration. 
The experiments of Mitchell and Eogers of Philadelphia have made it 
no longer doubtful that gases held in solution, and separated by porous 
membranes, are thereby in no degree exempted from the operation of 
this law. 
Pain in the ears and suspension of the faculty of hearing were 
caused by the pressure of a heavy column of air upon the membranes 
of the tympana, unsupported by equal counter pressure. After a short 
