64 
one who reflects upon the subject, and chooses to examine the specimen to which 
we have referred, will see at once that the range of action in the moving parts of 
an animal having the muscles inserted in an external crust or integument must he 
exceedingly limited; and that, in order to produce even a fraction of the different 
motions of which an animal with an internal skeleton is capable, there must he a 
much greater number both of articulations and of muscles. Accordingly, we find 
that the muscles of a caterpillar exceed by hundreds the muscles of a greyhound 
or an eagle ; and yet its motions are mere crawling as compared with theirs, and 
its body is a thing of no weight in comparison. 
In the human arm, or in any other limb having internal bones, the motion of 
a single joint may command a good deal more than an entire hemisphere, having 
the length of the articulated bone for its radius. This can be done in conse¬ 
quence of the real centre of motion in the joint of two internal bones being a 
point, which is equally affected to every plane passing through it, and, therefore, 
not tied down to any straight line crossing the direction of the articulated bones 
when they are straight. It is true that we never have in the body of any verte- 
bated animal this extreme variety of motion of which the joint of such an animal 
could be made capable ; because, joints being made for purposes, must, from the 
very nature of the case, have motion in the direction of their purpose, and a ful¬ 
crum or support for that motion in the opposite direction. This, however, does 
not affect the perfect universality of the principle ; and as motion on a single 
point, as a centre, is not affected by, or confined to, one direction more than ano¬ 
ther, there is an unlimited basis to the motions of vertebrated animals; and thus 
a joint can be formed capable of having its best motion in any direction that can 
be imagined. When several such joints are combined, the result is such as would 
stagger the belief of even those who are conversant with common mechanics, if 
they have not thought upon this very subject. Say that the human arm is, for 
instance, two feet in length, (making a little allowance for flexion in some posi¬ 
tions), and that it can command three-fourths of a sphere of two feet radius, which 
is within the truth, the human finger can, as told to the microscope, divide this 
space to the two-thousandth part of an inch every way; and as it must pass from 
one of these very proximate points to the other, it may absolutely be said to 
divide this space to infinitude—that is, to a degree of minuteness which we 
cannot express by numbers, and of which, in fact, we can have no conception. 
Add to this, the motion which the shoulder-joint can receive from the action of 
the rest of the body, and add to this the additional motion given by walking or 
running, or by the use of the feet generally—and the power of the finger in divid¬ 
ing space becomes an especial wonder, and should lead every one to employ, in 
the most useful manner, an instrument which has no parallel in the catalogue of 
material things. This subject is as long as it is instructive, and our limits are 
already exceeded; but we may resume it on some future occasion. 
