IN THE HUMAN FRAME. 
61 
siderably varied; but with a strict reference to the conveni- 
ency of the animal. [PI. IX. fig. 3, 4, 5] For, whereas in 
We often hear of a new ship going to sea to stretch her rigging; that 
is, to permit the shrouds and stays to be stretched by the motion of the 
ship, after which they are again braced tight; for if she were overtaken 
by a storm before this operation, and when the stays and shrouds were 
relaxed, the mast would lean against the upper deck, by which it would 
be sprung or carried away. Indeed, the greater proportion of masts that 
are lost are lost in this manner. There are no boats which keep the sea 
in such storms as those which navigate the gulf of Finland. Their masts 
are not attached at all to the hull of the ship, but simply rest upon the 
step. 
Although the spine has not a strict resemblance to the mast, the con¬ 
trivances of the ship-builder, however different from the provisions of na¬ 
ture, show what object is to be attained; and when we are thus made 
aware of what is necessary to the security of a column on a moveable 
base, we are prepared to appreciate the superior provisions of nature for 
giving security to the human spine. 
The human spine rests on what is called the pelvis , or basin;—a circle 
of bones, of which the haunches are the extreme lateral parts; and the sa¬ 
crum (which is the keystone of the arch) may be felt at the lower part 
of the back. To this central bone of the arch of the pelvis the spine is 
connected; and, taking the similitude of the mast, the sacrum is as the 
step on which the base of the pillar, like the heel of the mast, is socket¬ 
ed or morticed. The spine is tied to the lateral parts of the pelvis by 
powerful ligaments, which may be compared to the shrouds. They se¬ 
cure the lower part of the spine against the shock of lateral motion or 
rolling; but, instead of the stays to limit the play of the spine forwards 
and backwards in pitching, or to adjust the rake of the mast, there is a 
very beautiful contrivance in the'lower part of the column. 
The spine forms here a semicircle which has this effect; that whether 
by the exertion of the lower extremities, the spine is to be carried forward 
upon the pelvis, or whether the body stops suddenly in running, the jar 
which would necessarily take place at the lower part of the spine, if it 
stood upright like a mast, is distributed over several of the bones of the 
spine; and, therefore, the chance of injury at any particular part is di¬ 
minished. 
For example, the sacrum, or centre bone of the pelvis, being carried 
forward, as when one is about to run, the force is communicated to the 
lowest bone of the spine. But, then, the surfaces of these bones stand 
with a very slight degree of obliquity to the line of motion; the shock 
communicated from the lower to the second bone of the vertebrae is still in 
a direction very nearly perpendicular to its surface of contact. The same 
takes place in the communication of force from the second to the third, 
and from the third to the fourth; so that before the shock of the horizontal 
motion acts upon the perpendicular spine, it is distributed over four bones 
of that column, instead of the whole force being concentrated upon the 
joining of any two. 
If the column stood upright, it would be jarred at the lowest point of 
contact with its base. But by forming a semicircle, the motion would 
produce a jar on the very lowest part of the column, and which is distrib¬ 
uted over a considerable portion of the column; and in point of fact, this 
part of the spine never gives way. Indeed, we should be inclined to of- 
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