76 
OF THE MUSCLES. 
cles lying in straight direction. But whether the pull be sin¬ 
gle or combined, the articulation is always such, as to be 
capable of obeying the action of the muscles. The oblique 
muscles attached to the head, are likewise so disposed, as 
to be capable of the steadying the globe, as well as of mov¬ 
ing it. The head of a new-born infant is often obliged to 
be filleted up. After death, the head drops and rolls in 
every direction. So that it is by the equilibre of the mus¬ 
cles, by the aid of a considerable and equipollent muscular 
force in constant exertion, that the head maintains its erect 
posture. The muscles here supply what would otherwise 
be a great defect in the articulation; for the joint in the 
neck, although admirably adapted to the motion of the head, 
is insufficient for its support. It is not only by the means of 
a most curious structure of the bones that a man turns his 
head, but by virtue of an adjusted muscular power, that he 
even holds it up. 
As another example of what we are illustrating, viz. con¬ 
formity of use between the bones and the muscles, it has 
been observed of the different vertebrae, that their proces¬ 
ses are exactly proportioned to the quantity of motion which 
the other bones allow of, and which the respective muscles 
are capable of producing. 
II. A muscle acts only by contraction. Its force is ex¬ 
erted in no other way. When the exertion ceases, it relax¬ 
es itself, that is, it returns by relaxation to its former state; 
but without energy. This is the nature of the muscular 
fibre: and being so, it is evident that the reciprocal ener¬ 
getic motion of the limbs, by which we mean motion with 
force in opposite directions, can only be produced by the 
instrumentality of opposite or antagonist muscles; of flexors 
and extensors answering to each other. For instance, the 
biceps and brachiaeus internus muscles, placed in the front 
part of the upper arm,by their contraction bend the elbow; 
and with such degree of force, as the case requires, or the 
strength admits of. [PI. XIII. fig. 1.] The relaxation of 
these muscles, after the effort, would merely let the fore¬ 
arm drop down. For the back stroke, therefore, and that 
the arm may not only bend at the elbow, but also extend and 
straighten itself, with force, other muscles, the longus and 
brevis brachiaeus exlernus, and the anconaeus, placed on 
the hinder part of the arms, by their contractile twitch fetch 
back the fore-arm into a straight line with the cubit, with 
no less force than that with which it was bent out of it 
The same thing obtains in all the limbs, and in every mov¬ 
able part of the body. A finger is not bent and straighten- 
