200 
Proceedings of the Royal Irish Academy. 
The sequence of the phenomena constituting the Mid de Montagnes 
may be thus exphiiued : — 
In ascending high mountains, the great and rapid disintegration of 
muscle involved in the efforts demanded, renders increased rate of 
respiration necessary, partly to supply the additional oxygen required 
for the oxidation of the used-up tissues, and in part to cast off by the 
lungs the gaseous products of augmented chemical change. Again, 
the percentage of oxygen by volume in the air decreasing as its density, 
the number of inspiratory acts must be proportionately increased 
to supply the amount of oxygen required. 
Hence accelerated respiration ; but by a fixed physiological law 
the rate of circulation is definitely proportioned to that of breathing, 
and hence palpitation, quick pulse, and tiymitus aurium. 
The head symptoms — headach, vertigo, and somnolence — are in a 
measure due to the last-mentioned cause ; but in some degree likewise 
to the circulation in the sensorium, in the first instance, of imper- 
fectly decarbonized blood. 
JSTausea I regard as a reflex symptom, due to deranged intracranial 
circulation. 
The feeling of lassitude and depression of spirits, drowsiness, and 
superficial haemorrhages, are the direct result of deranged gaseous 
equilibrium ; the surface pressure being insufiicient to counterbalance 
that of the elastic fluids within the body. Blood is consequently 
forced to the surface on the same principle as by the operation of a 
cupping-glass, but in a minor degree. 
The effect of damp weather upon the spirits and bodily vigour 
is proverbial ; it is in part due to the gloomy aspect presented by 
nature enshrouded in mist ; but mainly to the accompanying reduc- 
tion in barometric pressure. 
How this operates upon the body so as to depress vital energy, is 
not well understood. My impression is that the effect in question 
is produced by lowered vascular pressure upon the nerve centres, 
consequent upon repletion of the superficial vessels. 
The unpleasant symptoms, mentioned as experienced by a person 
passing quickly, and with great muscular effort, into the higher regions 
of the atmosphere, are of very brief duration, and attended with no bad 
consequences, if the lungs and vascular system be healthy, and capable 
of bearing the strain involved in the sudden transition. 
On descending into the more dense strata of the air the traveller 
will experience a new series of phenomena, or rather the former series 
modified. The surplus pressure is now ah extra, and the symptoms 
consequent upon exposure to, and respiration of, compressed air, but 
in the relative sense only, are exhibited. 
Of the effects produced by the respiration of air absolutely com- 
pressed, and of greater density than that represented by the standard 
barometric pressure of thirty inches of mercury, comparatively little 
has been made known. This is mainly due to the absence in nature 
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