THE SEROUS MEMBRANES. 663 
tensive layers or mere bands, into which vessels speedily shoot, 
and the process of organization is accomplished. 
Extensibility .—This property of membrane generally is no¬ 
where more strikingly displayed than in the serous membranes, 
and particularly in that under consideration. How different the 
bulk of the lungs before the act of inspiration has commenced, 
and after it has been completed, and especially in the laborious 
respiration of disease or rapid exertion ! In either state of the 
lungs, the pleura seems to be perfectly fitted to that which it 
envelopes. 
Elasticity .—This property is connected with the preceding, 
and as clearly demonstrated. The power which distends it being 
withdrawn, it contracts with the diminished bulk of the lungs. 
It is one agent, and perhaps of no mean power, in the process of 
expiration. It is one illustration of that principle of elasticity 
to which, in the last lecture, I attributed the whole function of 
expiration. Accident occasionally presents us with a yet more 
evident proof of the elasticity or contractility of the investing 
membrane. If the parietes of the thorax are punctured, and the 
pressure of the atmosphere equalized within and without the 
pulmonary pleura, the lung collapses to less than half its size, 
even after the act of expiration had been completed. 
Vascularity .—In a state of health the bloodvessels of the 
pleura are not numerous. Considerable portions of the reflected 
part of it may be examined without any arteries or veins appear¬ 
ing, and where the blood will not follow the knife. Even the 
vessels that are apparent seem to be ramifying over it, rather than 
entering into it, or belonging to it. A membrane, the expansion 
of which is so variable, and indeed every moment changing, docs 
not seem to be calculated for the transmission of numerous 
vessels. The exhalents,. however, carrying the pellucid serous 
fluid of which I have been speaking, communicate with or pro¬ 
ceed from arterial ramifications ; and under inflammation there are 
vesselsenough, not previously containing red blood, which expand, 
and mark by their redness the seat and extent of inflammation. 
This is peculiarly evident in that patchy species of inflammation 
of the pleura sometimes accompanying rabies. 
Little Sensibility .—The pleura, like other serous membranes, 
is possessed of very little sensibility. Few nerves from the sen¬ 
sitive column of the spinal chord reach it. Acute feeling would 
render these membranes generally, and this membrane in par¬ 
ticular, unfit for the function they have to discharge. It has too 
much motion, even during sleep ; and far too forcible friction with 
the parietes of the thorax in morbid or hurried respiration, to ren¬ 
der it convenient or useful for it to possess much sensation. Some 
