112 
OF THE ANIMAL STRUCTURE 
fluid back to it again; the lungs performing their elaborate 
office, viz. distending and contracting their many thousand 
vesicles, by a reciprocation which cannot cease for a min¬ 
ute; the stomach exercising its powerful chemistry; the 
bowels silently propelling the changed aliment; collecting 
from it, as it proceeds, and transmitting to the blood an 
incessant supply of prepared and assimilated nourishment; 
that blood pursuing its course; the liver, the kidneys, the 
pancreas, the parotid, with many other known and dis¬ 
tinguishable glands, drawing off from it, all the while, 
their proper secretions. These several operations, togeth¬ 
er with others more subtile but less capable of being inves¬ 
tigated, are going on within us, at one and the same time. 
Think of this; and then observe how the body itself, the case 
which holds this machinery, is rolled, and jolted, and tossed 
about, the mechanism remaining unhurt, and with very 
little molestation, even ofits nicest motions. Observe a rope 
dancer, a tumbler, or a monkey: the sudden inversions and 
contortions which the internal parts sustain by the postures 
into which their bodies are thrown; or rather observe the 
shocks which these parts, even in ordinary subjects, some¬ 
times receive from falls and bruises, or by abrupt jerks and 
twists, without sensible, or with soon-recovered damage. 
Observe this, and then reflect how firmly every part must 
be secured, how carefully surrounded, how well tied down 
and packed together. 
This property of animal bodies has never, I think, been 
considered under a distinct head, or so fully as it deserves. 
I may be allowed, therefore, in order to verify my observa¬ 
tion concerning it, to set forth a short anatomical detail, 
though it oblige me to use more technical language than I 
should wish to introduce into a work of this kind. 
1. The heart (such care is taken of the centre of life) 
is placed between the soft lobes of the lungs; tied to the 
mediastinum and to the pericardium; which pericardium is 
not only itself an exceedingly strong membrane, but adheres 
firmly to the duplicature of the mediastinum, and, by its 
point, to the middle tendon of the diaphragm. The heart 
is also sustained in its place by the great blood-vessels which 
issue from it * 
2. The lungs are tied to the sternum by the mediasti¬ 
num, before; to the vertebrae by the pleura, behind. It 
seems indeed to be the very use of the mediastinum (which 
is a membrane that goes straight through the middle of the 
* Keill's Anat. p. 107. ed. tf. 
