1. THE MUSCLES.-- The great mass of the body external to the skeleton, is composed of the flesh, or _Muscles_, which largely determines its outline and weight. The muscles are the organs of motion. Their number is about four hundred, and to each of them is assigned a separate and distinct office. They have all been studied, one by one, and a name given to each, by the anatomist. Each is attached to bones which it is designed to move. A few are circular in form, and enclose cavities, the size of which they diminish by contraction. [Illustration: FIG. 8.--MUSCULAR TISSUE. _a_, _b_, Striped muscular fibres: _c_, The same more highly magnified.] [Sidenote: 2. The structure of flesh? Its color, etc.? The composition of the fibres? How marked?] 2. If we examine a piece of flesh, we observe that it is soft, and of a deep red color. Its structure appears to be composed of layers and bundles of small fibres. Let us further examine these fibres under the microscope. We now discover that they are, in turn, made up of still finer fibres, of _fibrillæ_: these are seen in Fig. 8. The fibres are beautifully {26} marked by parallel wavy lines, about ten thousand to an inch, which give the fibre its name of the _striped_ muscular fibre. All of the voluntary muscles present this appearance. [Sidenote: 3. Arrangement of the muscles? Their action? Flexion and extension? Action of the muscles when we stand erect?] 3. FLEXION AND EXTENSION.--The muscles are, for the most part, so arranged in pairs, or corresponding sets, that when motion is produced in one direction by one set, there is, opposite to it, another muscle, or group of muscles, which brings the limb back to its place. When they act alternately, a to-and-fro movement results. When a joint is bent, the motion is called _flexion_; and when it is made straight again, it is called _extension_. When both sets act equally, and at the same moment, no motion is produced, but the body or limb is maintained in a fixed position: this occurs when we stand erect. The muscles which produce extension are more powerful than those opposite to them. [Illustration: FIG. 9.--A, Biceps muscle of the arm: B, C, Its tendons.] [Sidenote: 4. Kinds of muscles? The voluntary? Involuntary? The heart? Give the example. The hand? Arm?] 4. The muscles are also distinguished, on the other hand, as the voluntary and involuntary muscles, according as they are, or are not, under the control of the will. The heart is an example of the involuntary variety. We cannot change its action in the least by an effort of the will. When we sleep, and the will ceases to act, the heart continues to beat without cessation. The voluntary muscles, on the other hand, are such as are used only when we wish or _will_ to use them--as the muscles of the hand or arm (Fig. 9). {27} [Sidenote: 5. What are the tendons or sinews? Their strength? Color? Location? Tendon of Achilles? The fable? Muscles of the leg?]