It is the principal Organ or Instrument of Motion; or it is a Portion of Flesh, wherein there are Veins, Arteries, Nerves, and Fibres, and which is cover'd with a Membrane. _How many parts are there in a Muscle?_ Three, _viz._ the Head, the Belly, and the Tail: The Head is that part thro' which the Nerve enters; the Belly is the Body or Middle of the Muscle; and the Tail is the Extremity, where all the Fibres of the Muscle are terminated to make the Tendon or String which is fasten'd to the Part whereto it gives Motion. {20} _Have all the Muscles their Fibres streight from the Head to the Tail?_ No, some have them streight, others transverse, and others oblique or circular, according to the several Motions to which they are appropriated. _How many sorts of Muscles are there with respecting to their Action?_ There are two different kinds, _viz._ the _Antagonists_ and the _Congenerate_; the former are those that produce opposite Motions; as a _Flexor_ and an _Extensor_, a _Depressor_ and a _Levator_. The Congenerate are those that contribute to one and the same Action; as when there are two Flexors or two Extensors, and then one supplies the defect of the other; whereas when one of the Antagonist Muscles is cut, the other becomes useless, and void of Action. _How is the Action of a Muscle perform'd?_ It is done by Contraction and Extension; the former causeth the Antagonist to swell, and the other compels it to stretch forth in length. _What is _Aponeurosis_?_ It is the continuity of the Fibres of a Tendon which makes a Connexion that serves to strengthen the Muscle in its Motion. * * * * * {21} CHAP VII. _Of the Myology, or Anatomy of the Muscles of the Head._ _How many Muscles are there appointed to move the Head, and which be they?_ The Head is mov'd by the means of fourteen Muscles, seven on each side; of these, two serve to depress it, eight to lift it up, and four to turn it round about. The two Depressors are call'd _Sternoclinomastoidei_; they take their Rise in the _Sternum_, at the Clavicles, and proceed obliquely to join the _Apophysis Mastoides_. Of the four Elevators on each side the first is the _Splenius_, which begins at the five _Vertebræ_ of the Back and the three lower ones of the Neck, and ascending obliquely, cleaves to the hinder part of the Head. The second, named _Complexus_ or _Trigeminus_, having its beginning as the _Splenius_, sticks in like manner to the hinder part of the Head, and they form together a figure resembling that of S. _Andrew_'s Cross. The third is the _Rectus Major_, which proceeding from the second _Vertebra_ of the Neck, shoots forward to join the hinder part of the Head. The fourth is the _Rectus Minor_, which begins at the first _Vertebra_ of the Neck, and ends likewise in the hinder part of the Head.