The two kinds of muscles, then, are the red, voluntary, striated muscles, and the smooth, involuntary, non-striated muscles.

66.  Structure of Voluntary Muscles.  The main substance which clothes the bony framework of the body, and which forms about two-fifths of its weight, is the voluntary muscular tissue.  These muscles do not cover and surround the bones in continuous sheets, but consist of separate bundles of flesh, varying in size and length, many of which are capable of independent movement.

Each muscle has its own set of blood-vessels, lymphatics, and nerves.  It is the blood that gives the red color to the flesh.  Blood-vessels and nerves on their way to other parts of the body, do not pass through the muscles, but between them.  Each muscle is enveloped in its own sheath of connective tissue, known as the fascia.  Muscles are not usually connected directly with bones, but by means of white, glistening cords called tendons.

[Illustration:  Fig. 30.—­Striated (voluntary) Muscular Fibers.

  A, fiber serparating into disks;
  B, fibrillae (highly magnified);
  C, cross section of a disk
]

If a small piece of muscle be examined under a microscope it is found to be made up of bundles of fibers.  Each fiber is enclosed within a delicate, transparent sheath, known as the sarcolemma.  If one of these fibers be further examined under a microscope, it will be seen to consist of a great number of still more minute fibers called fibrillae.  These fibers are also seen marked cross-wise with dark stripes, and can be separated at each stripe into disks.  These cross markings account for the name striped or striated muscle.

The fibrillae, then, are bound together in a bundle to form a fiber, which is enveloped in its own sheath, the sarcolemma.  These fibers, in turn, are further bound together to form larger bundles called fasciculi, and these, too, are enclosed in a sheath of connective tissue.  The muscle itself is made up of a number of these fasciculi bound together by a denser layer of connective tissue.

Experiment 17. To show the gross structure of muscle. Take a small portion of a large muscle, as a strip of lean corned beef.  Have it boiled until its fibers can be easily separated.  Pick the bundles and fasciculi apart until the fibers are so fine as to be almost invisible to the naked eye.  Continue the experiment with the help of a hand magnifying glass or a microscope.

67.  The Involuntary Muscles.  These muscles consist of ribbon-shaped bands which surround hollow fleshy tubes or cavities.  We might compare them to India rubber rings on rolls of paper.  As they are never attached to bony levers, they have no need of tendons.

[Illustration:  Fig. 31.—­A, Muscular Fiber, showing Stripes, and Nuclei, b and c. (Highly magnified.)]

The microscope shows these muscles to consist not of fibers, but of long spindle-shaped cells, united to form sheets or bands.  They have no sarcolemma, stripes, or cross markings like those of the voluntary muscles.  Hence their name of non-striated, or unstriped, and smooth muscles.