THE GENERAL STRUCTUILE. 131 



US at one and the same time. Think of this ; and tlien ob- 

 serve how the body itself, the case which holds this machine- 

 ry, is rolled, and jolted, and tossed about, the mechanism 

 remaining unhurt, and with very little molestation even of 

 its 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, sometimes 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 sur- 

 rounded, 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 two soft lobes of the lungs ; is tied to 

 the mediastmum and to the pericardium ; which pericardi- 

 um 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 bloodvessels 

 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 tho- 

 rax, from the breast to the back — to keep the contents ol 

 the thorax in their places ; in particular to hinder one lobe 

 of the lungs from incommoding another, or the parts of the 

 lungs from pressing upon each other when we lie on one side.t 



* Keill's Anat., p. 107, ed. 3. t lb., p. 119, ed. 3. 



