70 OF THE MUSCLKS. 



to the head, arc likewise so disposed, as to be capable of 

 steadying the jrlobe, as well as ofmovinj: it. The head ol' 

 a new-born ini'ant is oitcn obliged to l>e lilietted up. After 

 death the head drops and rolls in every direction. So that 

 it is by the equilibre of the muscles, by the aid of a consid- 

 erable and equipollent muscular force in constant exertion, 

 that the liead 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 lie 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 

 beeji observed of the different vertebra;, 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 otiier way. When the exertion ceases it relax- 

 es itself; Uiat 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 (ncr- 

 ffdic motion of the limbs, by which we mean motion initk 

 force in opj)Osite directions can only be produced by the 

 instrumentality of oi)posite or antagonist muscles ; of fiexors 

 and extensors answering to each other. For instance, the 

 biceps and brachia-us rnfcrm/s 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 (IM. XIll. fig. 1.) The relaxation of 

 these muscles, after tho effort, would merely let tho fore- 

 arm drop down. For tlic bark stro/:e, therefore, and that 

 the arm may not only bend at the elbow, but also extend and 

 straighten itself with fcrcn, other muscles, tlie longus and 

 brevis brachia-us r/frrnu.-i, and the ancon.rMis, placed on 

 the hinder part of the arm, by their contractile twitch fctcii 

 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 move- 

 able part of the body. A finger is not bent and straighten- 

 ed, without the contraction of two muscles taking place, it 

 is evident, therefore, that the ;inimal Junctions retpiire that 



