84 
OF THE MUSCLES 
body by the smallest muscles; and he mentions, as his 
examples, the muscles which have been discovered in the 
iris of the eye, and the drum of the ear. The tenuity of 
these muscles is astonishing.* They are microscopic hairs; 
must be magnified to be visible; yet they are real, effective 
muscles; and not only such, but the grandest and most pre¬ 
cious of our faculties, sight and hearing, depend upon their 
health and action. 
V. The muscles act in the limbs with what is called a 
mechanical disadvantage. The muscle at the shoulder, 
[PI. XIII. fig. 1. f.] by which the arm is raised, is fixed 
nearly in the same manner as the load is fixed upon a 
steelyard, within a few decimals, we will 3ay, of an inch, 
from the centre upon which the steelyard turns. In this 
situation, we find that a very heavy draught is no more 
than sufficient to countervail the force of a small lead 
plummet, placed upon the long arm of the steelyard, at the 
distance of perhaps fifteen or twenty inches from the cen¬ 
tre, and on the other side of it. And this is the disadvantage 
which is meant. And an absolute disadvantage, no doubt, 
would be, if the object were to spare the force of muscu- 
dr contraction. But observe how conducive is this consti¬ 
tution to animal conveniency. Mechanism has always in 
view one or other of these two purposes; either to move a 
great weight slowly, and through a small space; or to move 
a light weight rapidly, through a considerable sweep. For 
the former of these purposes, a different species of lever, 
and a different collocation of the muscles, might be better 
than the present; but for the second, the present structure 
is the true one. Now so it happens, that the second, and 
not the first, is that which the occasions of animal life prin¬ 
cipally call for. In what concerns the human body, it is 
of much more consequence to any man to be able to carry 
his hand to his head with due expedition, than it would be 
to have the power of raising from the ground a heavier load 
(of two or three more hundred weight we will suppose,) 
than he can lift at present. This last i^a faculty, which on 
some extraordinary occasions he may desire to possess, 
but the other is what he wants, and uses every hour and 
minute. In like manner, a husbandman, or a gardener, 
will do more execution, by being able to carry his scythe, 
his rake, or his flail, with a sufficient despatch through a 
sufficient space, than if, with greater strength, his motions 
were proportionably more confined and slow. It is the 
same with a mechanic in the use of his tools. It is the 
same also with other animals in the use of their limbs. In 
