56 
OF MECHANICAL ARRANGEMENT 
difficulty of accomplishing the task, before he be told how 
the same thing is effected in the animal frame. Nothing 
will enable him to judge so well of the wisdom which has 
been employed; nothing will dispose him to think of it so 
truly. First, for the firmness, yet flexibility, of the spine, 
it is composed of a great number of bones (in the human 
subject, of twenty-four) joined to one another, and compact¬ 
ed by broad bases. The breadth of the bases upon which 
the parts severally rest, and the closeness of the junction, 
give to the chain its firmness and stability; the number of 
parts, and consequent frequency of joints, its flexibility. 
Which flexibility, we may also observe, varies in different 
parts of the chain: is least in the back, where strength 
more than flexure, is wanted; greater in the loins, which 
it was necessary should be more supple than the back, 
and greatest of all in the neck, for the free motion of the 
head. Then, secondly, in order to afford a passage for 
the descent of the medullary substance, each of these 
bones is bored through the middle in such a manner, as 
that, when put together, the hole in one bone falls into a 
line, and corresponds with the holes in the two bones con¬ 
tiguous to it. By which means the perforated pieces, 
when joined, form an entire, close, uninterrupted channel; 
at least, whilst the spine is upright, and at rest. But, as 
a settled posture is inconsistent with its use, a great diffi¬ 
culty still remained, which was to prevent the vertebrae 
shifting upon one another, so as to break the line of the 
canal as often as the body moves or twists; or the joints 
gaping externally, whenever the body is bent forward, and 
the spine thereupon made to take the form of a bow. These 
dangers, which are mechanical, are mechanically provided 
against. The vertebrae, by means of their processes and 
projections, and of the articulations which some of these 
form with one another at their extremities, are so locked in 
and confined, as to maintain, in what are called the bodies or 
broad surfaces of the bones, the relative position nearly un¬ 
altered; and to throw the change and the pressure, produced 
by flexion, almost entirely upon the intervening cartilages, 
the springiness and yielding nature of whose substance ad¬ 
mits of all the motion which is necessary to be performed 
upon them, without any chasm being produced by a separa¬ 
tion of the parts. I say, of all the motion which is necessa¬ 
ry; for although we bend our backs to every degree almost 
of inclination, the motion of each vertebrce is very small: 
such is the advantage we receive from the chain being 
composed of so many links, the spine of so many bones. 
