150. The MUSCLES, by their size and number, constitute the great bulk of the body, upon which they bestow form and symmetry. In the limbs, they are situated around the bones, which they invest and defend, while they form, to some of the joints, their principal protection. In the trunk, they are spread out to enclose cavities, and constitute a defensive wall, capable of yielding to internal pressure, and reassuming its original state. 151. In structure, a muscle is composed of _fas-cic´u-li_ (bundles of fibres) of variable size. These are enclosed in a cellular membranous investment, or sheath. Every bundle composed of a number of small fibres, and each fibre consists of a number of filaments, each of which is enclosed in a delicate sheath. Toward the extremity of the organ the muscular fibre ceases, and the cellular structure becomes aggregated, and so modified as to constitute _ten´dons_, (cords,) by which the muscle is tied to the surface of the bone. The union is so firm, that, under extreme violence, the bone will sooner break than permit the tendon to separate from its attachment. In some situations, there is an expansion of the tendon, in the manner of a membrane, called _Ap-o-neu-ro´sis_, or _Fas´ci-a_. -=-=-=-=-=-=-=-=-=-=-=-= 149. How are all the motions of the body produced? What are these motor organs called? 150-160. _Give the anatomy of the muscles._ 150. What is said of the muscles? 151. Give their structure. -=-=-=-=-=-=-=-=-=-=-=-= _Observation._ The pupil can examine a piece of boiled beef, or the leg of a fowl, and see the structure of the fibres and tendons of a muscle. [Illustration: Fig. 36. 1, A representation of the direction and arrangement of the fibres in a fusiform, or spindle-shaped muscle. 2, In a radiated muscle. 3, In a penniform muscle. 4, In a bipenniform muscle. _t_, _t_, The tendons of a muscle.] 152. Muscles present various modifications in the arrangement of their fibres, as relates to their tendinous structure. Sometimes they are completely longitudinal, and terminate, at each extremity, in a tendon, the entire muscle being spindle-shaped. In other situations, they are disposed like the rays of a fan, converging to a tendinous point, and constituting a _ra´di-ate_ muscle. Again they are _pen´ni-form_, converging, like the plumes of a pen, to one side of a tendon, which runs the whole length of the muscle; or they are _bi-pen´ni-form_, converging to both sides of the tendon. -=-=-=-=-=-=-=-=-=-=-=-= How are tendons or cords formed? What is the expansion of a tendon called? How can the structure of muscles and their fibres be shown? What does fig. 36 represent? 152. Give the different arrangements of muscular fibres. -=-=-=-=-=-=-=-=-=-=-=-=