6-30 
ON EMPHYSEMA OF THE LUNGS. 
from the fulfilment of a law in the animal economy which 
establishes a constant proportion between the quantity of blood 
to be aerated in a given time, and the extent of surface on which 
this aeration is to be accomplished. Thus we find that the lung' 
has its maximum of density in infants, and in those animals 
that have either a very rapid circulation or a very large supply 
of blood ; and that, on the contrary, the density of the lung is 
at its minimum in old persons, and in such animals as receive 
into their lungs, at each round of the circulation, only a small 
proportion of the blood contained in their circulating system*. 
Besides these, which may be considered as the physiological 
causes of pulmonary emphysema, there are others which pro¬ 
duce a similar condition of the organ in a manner purely me¬ 
chanical, by keeping the air-cells in a state of over-distention. 
Amongst these may be enumerated violent efforts of any kind 
(especially if often renewed) which cause the long-continued 
retention of the breath ; repeated attacks of catarrh, bronchitis, 
asthma, or other diseases of the lungs or air-tubes, attended 
with difficulty of breathing, or distressing paroxysms of coughing. 
The mechanism of the over-distention and rupture of the air- 
cells in these cases may be explained by the efforts which are 
constantly made by the powerful muscles of inspiration to in¬ 
troduce a fresh supply of air into the air-cells, while that which 
they contain is prevented from escaping by pellets of viscid 
mucus, spasmodic stricture of the bronchi, or turgescence of the 
bronchial membrane, according to the nature of the disease 
which produced the dyspnoea. In this way, the air-cells are 
kept in a state of over-distention, which the efforts that are made 
to evacuate them only tend to confirm and increase; and pro¬ 
vided the obstruction is of some continuance, the dilated con¬ 
dition of the cells will be rendered permanent, or else their 
parietes will give way, and allow several cells to be thrown into 
one. 
From this view of the matter, it will readily be understood 
why all diseases accompanied by protracted attacks of dyspnoea, 
or violent and often repeated paroxysms of coughing, are so 
constantly followed by emphysema, especially when occurring 
m persons advanced in life, in w'hom, as we have already ex¬ 
plained, the lungs are peculiarly predisposed to this disease by 
the atrophy which their parenchymatous texture naturally under¬ 
goes at this period of life. But though the over-distention and 
rupture of the air-cells is in general a slow process produced by 
long repeated efforts to overcome an obstruction to the free exit 
of their contents, and is consequently the result, in most cases, 
** Andral , Op. Cit. 
