*Striated Muscle Cells.*-The cells of the striated muscles are slender, thread-like structures, having an average length of 1-1/2 inches (35 millimeters) and a diameter of about 1/400 of an inch (60 ). Because of their great length they are called fibers, or fiber cells. They are marked by a number of dark, transverse bands, or stripes, called striations,(83) which seem to divide them into a number of sections, or disks (Fig. 108).

A thin sac-like covering, called the _sarcolemma_, surrounds the entire cell and just beneath this are a number of nuclei.(84)

[Fig. 108]

Fig. 108-*A striated muscle cell* highly magnified, showing striations and nuclei. Attached to the cell is the termination of a nerve fiber.

Within the sarcolemma are minute fibrils and a semiliquid substance, called the _sarcoplasm_. At each end the cell tapers to a point from which the sarcolemma appears to continue as a fine thread, and this, by attaching itself to the inclosing sheath, holds the cell in place. Most of the muscle cells receive, at some portion of their length, the termination of a nerve fiber. This penetrates the sarcolemma and spreads out upon a kind of disk, having several nuclei, known as the _end plate_.

*The "Muscle-organ."*-We must distinguish between the term "muscle" as applied to the muscular tissue and the term as applied to a working group of muscular tissue, which is an organ. In the muscle, or muscle-organ, is found a definite grouping of muscle fibers such as will enable a large number of them to act together in the production of the same movement. An examination of one of the striated muscles shows the individual fibers to lie parallel in small bundles, each bundle being surrounded by a thin layer of connective tissue. (See Practical Work.) These small bundles are bound into larger ones by thicker sheaths and these in turn may be bound into bundles of still larger size (Fig. 109). The sheaths surrounding the fiber bundles are connected with one another and also with the outer covering of the muscle, known as

[Fig. 109]

Fig. 109-*Diagram* of a section of a muscle, showing the perimysium and the bundles of fiber cells.

[Fig. 110]

Fig. 110-*A muscle-organ in position.* The tendons connect at one end with the bones and at the other end with the fiber cells and perimysium. (See text.)

*The Perimysium.*-The plan of the muscle-organ is revealed through a study of the perimysium. This is not limited to the surface of the muscle, as the name suggests, but properly includes the sheaths that surround the bundles of fibers. Furthermore, the surface perimysium and that within the muscle are both continuous with the strong, white cords, called _tendons_, that connect the muscles with the bones. By uniting with the bone at one end and blending with the perimysium and fiber bundles at the other, the tendon forms a very secure attachment for the muscle. The perimysium and the tendon are thus the means through which the fiber cells in any muscle-organ are made to _pull together_ upon the same part of the body (Fig. 110).

*Purpose of Striated Muscles.*-The striated muscles, by their attachments to the bones, supply motion to all the mechanical devices, or machines, located in the skeleton. Through them the body is moved from place to place and all the external organs are supplied with such motion as they require. Because of the attachment of the striated muscles to the skeleton, and their action upon it, they are called _skeletal_ muscles. As most of them are under the control of the will, they are also called _voluntary_ muscles. They are of special value in adapting the body to its surroundings.

*Structure of the Non-striated Muscles.*-The cells of the non-striated muscles differ from those of the striated muscles in being decidedly spindle-shaped and in having but a single well-defined nucleus (Fig. 111).

Furthermore, they have no striations, and their connection with the nerve fibers is less marked. They are also much smaller than the striated cells, being less than one one-hundredth of an inch in length and one three-thousandth of an inch in diameter.

In the formation of the non-striated muscles, the cells are attached to one another by a kind of muscle cement to form thin sheets or slender bundles. These differ from the striated muscles in several particulars.

They are of a pale, whitish color, and they have no tendons. Instead of being attached to the bones, they usually form a distinct layer in the walls of small cavities or of tubes (Fig. 111). Since they are controlled by the part of the nervous system which acts independently of the will, they are said to be _involuntary_. They contract and relax slowly.

[Fig. 111]

Fig. 111-*Non-striated muscle cells.* _A._ Cross section of small artery magnified, showing (1) the layer of non-striated cells. _B._ Three non-striated cells highly magnified.

*Work of the Non-striated Muscles.*-The work of the non-striated muscles, both in purpose and in method, is radically different from that of the striated. They do not change the _position_ of parts of the body, as do the striated muscles, but they alter the _size_ and _shape_ of the parts which they surround. Their purpose, as a rule, is to move, or control the movement of, materials within cavities and tubes, and they do this by means of the _pressure_ which they exert. Examples of their action have already been studied in the propulsion of the food through the alimentary ca.n.a.l and in the regulation of the flow of blood through the arteries (pages 159 and 49). While they do not contract so quickly, nor with such great force as the striated muscles, their work is more closely related to the vital processes.

*Structure of the Heart Muscle.*-The cells of the heart combine the structure and properties of the striated and the non-striated muscle cells, and form an intermediate type between the two. They are cross-striped like the striated cells, and are nearly as wide, but are rather short (Fig. 112). Each cell has a well-defined nucleus, but the sarcolemma is absent. They are placed end to end to form fibers, and many of the cells have branches by which they are united to the cells in neighboring fibers. In this way they interlace more or less with each other, but are also cemented together. They contract quickly and with great force, but are not under control of the will. Muscular tissue of this variety seems excellently adapted to the work of the heart.

[Fig. 112]

Fig. 112-*Muscle cells from the heart*, highly magnified (after Schafer).

*The Muscular Stimulus.*-The inactive, or resting, condition of a muscle is that of relaxation. It does work through contracting. It becomes active, or contracts, only when it is being acted upon by some force outside of itself, and it relaxes again when this force is withdrawn. Any kind of force which, by acting on muscles, causes them to contract, is called a _muscular stimulus_. Electricity, chemicals of different kinds, and mechanical force may be so applied to the muscles as to cause them to contract. These are _artificial_ stimuli. So far as known, muscles are stimulated _naturally_ in but one way. This is through the nervous system.

The nervous system supplies a stimulus called the _nervous impulse_, which reaches the muscles by the nerves, causing them to contract. By means of nervous impulses, all of the muscles (both voluntary and involuntary) are made to contract as the needs of the body for motion require.

*Energy Transformation in the Muscle.*-The muscle serves as a kind of engine, doing work by the transformation of potential into kinetic energy.

Evidences of this are found in the changes that accompany contraction.

Careful study shows that during any period of contraction oxygen and food materials are consumed, waste products, such as carbon dioxide, are produced, and heat is liberated. Furthermore, the _blood supply to the muscle_ is such that the materials for providing energy may be carried rapidly to it and the products of oxidation as rapidly removed. Blood vessels penetrate the muscles in all directions and the capillaries lie very near the individual cells (Fig. 113). Provision is made also, through the nervous system, for _increasing_ the blood supply when the muscle is at work. From these facts, as well as from the great force with which the muscle contracts, one must conclude that the muscle is a _transformer of energy_-that within its protoplasm, chemical changes take place whereby the potential energy of oxygen and food is converted into the kinetic energy of motion.

[Fig. 113]

Fig. 113-*Capillaries* of muscles.

*Plan of Using Muscular Force.*-Two difficulties have to be overcome in the using of muscular force in the body. The first of these is due to the fact that the muscles exert their force _only when they contract_. They can pull but not push. Hence, in order to bring about the opposing movements(85) of the body, each muscle must work against some force that produces a result directly opposite to that which the muscle produces.

Some of the muscles (those of breathing) work against the elasticity of certain parts of the body; others (those that hold the body in an upright position), to some extent against gravity; and others (the non-striated muscle in arteries), against pressure. But in most cases, _muscles work against muscles_.

[Fig. 114]

Fig. 114-*The muscle pair* that operates the forearm. For names of these muscles, see Fig. 119.

The striated, or skeletal, muscles are nearly all arranged after the last-named plan. As a rule a pair of muscles is so placed, with reference to a joint, that one moves the part in one direction, and the other moves it in the opposite direction. From the kinds of motion which the various muscle pairs produce, they are cla.s.sified as follows:

1. _Flexors and Extensors._-The flexor muscles bend and the extensors straighten joints (Fig. 114).

2. _Adductors and Abductors._-The adductors draw the limbs into positions parallel with the axis of the body and the abductors draw them away.

3. _Rotators_ (two kinds).-The rotators are attached about pivot joints and bring about twisting movements.

4. _Radiating and Sphincter Muscles. _-The radiating muscles open and the sphincter muscles close the natural openings of the body, such as the mouth.

The pupil should locate examples of the different kinds of muscle pairs in his own body.

*Exchange of Muscular Force for Motion.*-The second difficulty to be overcome in the use of muscular force in the body is due to the fact that the muscles contract through _short_ distances, while it is necessary for most of them to move portions of the body through _long_ distances. It may be easily shown that the longest muscles of the body do not shorten more than three or four inches during contraction. To bring about the required movements of the body, which in some instances amount to four or five feet, requires that a large proportion of the muscular force be exchanged for motion. The machines of the skeleton, while providing for motion in definite directions, also provide the means whereby _strong forces_, acting through _short distances_, are made to produce movements of _less force_, through _long distances_. The mechanical device employed for this purpose is known as

*The Lever.*-The lever may be described as a stiff bar which turns about a fixed point of support, called the _fulcrum_. The force applied to the bar to make it turn is called the _power_, and that which is lifted or moved is termed the _weight_. The weight, the power, and the fulcrum may occupy different positions along the bar and this gives rise to the three kinds of levers, known as levers of the first cla.s.s, the second cla.s.s, and the third cla.s.s (Fig. 115). In levers of the _first cla.s.s_ the fulcrum occupies a position somewhere between the power and the weight. In the _second cla.s.s_ the weight is between the fulcrum and the power. In the _third cla.s.s_ the power is between the fulcrum and the weight.

[Fig. 115]

Fig. 115-*Cla.s.ses of levers. I.* Two levers of first cla.s.s showing fulcrums in different positions. II. Lever of second cla.s.s. III. Lever of third cla.s.s. _F._ Fulcrum. _P._ Power. _W._ Weight. _a._ Power-arm. _b._ Weight-arm.

*Application to the Body.*-In the body the bones serve as levers; the turning points, or fulcrums, are found at the joints; the muscles supply the power; and parts of the body, or things to be lifted, serve as weights. For these levers to _increase_ the motion of the muscles, it is necessary that the muscles be attached to the bones _near the joints_, and that the parts to be moved be located at some distance from the joints. In other words the (muscle) power-arm must be _shorter_ than the (body) weight-arm.(86)

Examining Fig. 116, it is seen that the distances moved by the power and weight vary as their respective distances from the fulcrum. That is to say, if the weight is twice as far from the fulcrum as the power, it will move through twice the distance, and if three times as far, through three times the distance. Thus the muscles, by acting through short distances (on the short arms of levers), are able to move portions of the body (located on the long arms) through long distances. Can all three cla.s.ses of levers be used in this way in the body?