548 
ON THE THORAX, RIBS, &C. 
The Thorax adapts itself to the changing bulk oj its contents .— 
A still more important consideration with regard to the parietes 
of the thorax is the manner in which they can adapt themselves 
to the changing bulk of the contents of the cavity. The capa¬ 
city of the chest is little affected by the alternate contraction and 
dilatation of the heart, for when its ventricles are collapsed its 
auricles are distended, and when its auricles are compressed its 
ventricles expand; but with regard to the lungs, it is a very 
different affair. In their state of collapse and expansion, they 
are of very different size ; they vary in comparative bulk one- 
sixth part or more; and in either state it is necessary for the pro¬ 
per discharge of the function of respiration that the parietes of 
the chest should be in contact with them. 
Number of the Ribs. —Let us consider this a little. The ribs 
are eighteen in number on either side. Nine of them are per¬ 
fect, and commonly called the true —I would rather say sternal, 
ribs, extending from the spine to the sternum; the remaining 
nine are posterior and shorter, and are only indirectly connected 
with the sternum. The number is sometimes variable. Not 
unfrequently there are nineteen on each side, but the additional 
ones are always posterior ribs. In this thorax you can plainly 
count twenty-one on each side, but three of these are supplemen¬ 
tary. There is a double curvature in the spine, and, occupying 
and supporting each of the arches, are three additional, perfect, 
but not very stout ribs, placed within the natural ones, which 
retain their usual situation. It is one of the most beautiful illus¬ 
trations I have ever seen of that presiding power, that vis medi- 
catrix naturae, which is ever employed in preserving the healthy 
structure and function of the frame, or resorting to the most ad¬ 
mirable contrivances to repair injury, or counteract disease. 
These things have hitherto been strangely overlooked among us. 
How connected with the Spine. —These ribs are united to the 
corresponding vertebrae or bones of the spine, so as to form per¬ 
fect joints, or rather each rib forms two joints. The head of the 
rib is received between the vertebrae or bone of the spine, above 
and below, so that it shall always present two articulating sur¬ 
faces, one opposed to each of the vertebrae, and both forming one 
joint, with a perfect capsular ligament, and admitting of a rota¬ 
tory motion. The head of the rib seems to be received, as it 
were, into the cartilago-ligamentous substance between the ver¬ 
tebrae. 
The Second Joint. —A little below is a turberosity which arti¬ 
culates with the inferior surface of the transverse process of the 
lower of the two vertebrae with which the head is united. This 
surface varies in the different vertebrae. In the anterior ones it 
