THE MUSCULAR TISSUES.
The voluntary muscles are for the most part placed in close relation with the skeleton, being attached to the hard parts, and moving these in different directions by their contraction.
The muscles are all symmetrical, and with the exception of the sphincters and one or two others are in pairs. Each muscle constitutes a separate organ, composed chiefly of contractile fibrous tissue, which is called muscular, and of other tissues and parts which may be regarded as accessory. Thus muscular fibers are connected together in bundles or fasciculi, and these fasciculi are again embedded in and united together by a quantity of connective tissue, forming the perimysium; and the whole is usually inclosed in an external sheath of the same material. Many of the muscles are connected at their more or less tapering extremities with tendons by which they are attached to the bones or hard parts; and the tendinous bands frequently run to a considerable length either on the surface of the muscle or between its fibers.
There are two chief kinds of muscular tissue, the striped, and the plain or unstriped, and they are distinguished by structural peculiarities and mode of action. The striped form of muscular fibers is sometimes called voluntary muscle, because all muscles under the control of the will are constructed of it. The plain or unstriped variety is often termed involuntary, because it alone is found in the greater number of muscles over which the will has no power.
The involuntary or unstriped muscles are made up of elongated, spindle-shaped fiber cells, which in their most perfect form are flat, from about 1⁄4500 to 1⁄3500 of an inch broad, and about 1⁄600 to 1⁄300 of an inch in length; very clear, and granular and brittle so that when they break they often have abruptly rounded or square extremities.
The fibers of involuntary muscles form the proper muscular coats of the digestive canal, æsophagus, urinary bladder, trachea, bronchi, gall-bladder, blood-vessels, lymphatics, etc. To this kind of fiber, muscular fiber, the term organic is often applied. The sympathetic or ganglionic portion of the nervous system, which consists of a chain of ganglia connected by nervous cords, extends from the cranium to the pelvis, along each side of the vertebral column, and from which nerves with ganglia proceed to the viscera in the thoracic, abdominal, and pelvic cavities. By its distribution, as well as by its peculiar mode of action, this system is less immediately connected with the mind, as conducting either sensation or the impulses of the will; it is more closely connected than the cerebro-spinal system is with the processes of organic life.
The muscles of animal life, or striped muscles, include the whole class of voluntary muscles, the heart, and those muscles neither completely voluntary nor completely involuntary, etc. All these muscles are composed of fleshy bundles called fasciculi, inclosed in coverings of fibro-cellular tissue, by which each is at once connected with, and isolated from, those adjacent to it. Each bundle is again divided into smaller ones similarly ensheathed and similarly divisible; and so on, through an uncertain number of gradations, till we arrive at the primitive fasciculi or the muscular fibers peculiarly so called.
Muscular fibers consist each of a tube or sheath of delicate structureless membrane, inclosing a number of filaments or fibrils. They are of cylindrical form, or of prismatic with one or more sides, according to the manner in which they are compressed by adjacent tissues. Their average diameter is about 1⁄500 of an inch, and their length never exceeds an inch and a half.
The arrangement of the elementary substances in a muscular fiber (the sarcos element or protoplasm inclosed in the sarcolemna, the sheath) composing a muscular fiber may be compared to Volta’s pile or an electric battery. In fact, both muscle and nerve are made up of electrical molecules, each of the two ends of which is negative—though the development of the electrical current is at present very imperfectly known.
Besides, there is every reason to believe that the ground substance is similar in nature to ordinary protoplasm, but without the granular character commonly but not always exhibited.
Blood-vessels are largely distributed in the substance of a muscle, carrying the materials necessary for its nourishment and chemico-vital changes, and there are also lymphatic vessels as in other vascular parts of the body.
Nerves run through every muscle, by which the muscular contractions are called forth, and a low degree of muscular sensibility is conferred upon the muscular substance.
The blood-vessels of the muscular tissues are extremely abundant, so that when they are successfully filled with a colored injection the fleshy parts of the muscle contrast strongly with its tendons. The arteries, accompanied by their veins, enter the muscle at various points and divide into branches, etc.
The nerves of a voluntary muscle are of considerable size. Their branches pass between the fasciculi and repeatedly unite with each other in form of a plexus, which is for the most part confined to a small part of the length of the muscle, or muscular division, in which it lies.
The voluntary muscles to which distinct names have been given in the system amount to about 240, and they naturally fall under the following four great divisions (the muscles are symmetrical and with few exceptions are in pairs):
| A. In the axial part of the body: | ||
| 1. Muscles of the head and neck, | 75 | |
| 2. Muscles of the vertebral column and trunk, | 51 | |
| B. In the limbs: | ||
| 3. Muscles of the upper extremities, | 58 | |
| 4. Muscles of the lower extremities, | 59 | |
Flesh and blood have nearly the same ultimate composition. On evaporating 1000 parts of blood it yields 790 parts of water and 210 parts solid residue. The elements that enter into the composition of the solid matter are as follows:
| Flesh. | Blood. | |
| Carbon, | 51.86 | 51.96 |
| Hydrogen, | 7.58 | 7.25 |
| Nitrogen, | 15.03 | 15.07 |
| Oxygen, | 21.30 | 21.30 |
| Ashes, | 4.23 | 4.43 |
The general composition of a human muscle is shown by the following table:
| Water, | 744 | .5 | ||||
| Solids. |  | Myosin and other matters, elastic elements, etc. | 155 | .4 | ||
| Soluble elements, | 19 | .3 | ||||
| Gelatine, | 20 | .7 | ||||
| Extractives, | 37 | .1 | ||||
| Fats, | 23 | .02 | 55 | .5 | ||
The muscles of the flesh form a large proportion of the weight of the whole body. Calculated for a man of 150 pounds’ weight:
| The skeleton, bone, | 27 | lbs. |
| The muscles, | 63 | lbs.,, |
| The viscera, with skin, fat, blood, etc., | 60 | lbs.,, |
The property of muscular tissue by which its peculiar functions are exercised, is its contractility—contraction or shortening. This is excited by all kinds of stimuli, applied either directly to the muscles, or indirectly to them through the medium of their nerves.
The muscular tissues perform all the physical work—as locomotion, every kind of action and exertion—of the body.
The quantity of blood circulated through the body is estimated to be from about 1⁄10 to about 1⁄13 part of the body’s weight, and about ¼ of that is distributed in the muscles.
As regards the action of the muscles the following general principles ought to be kept in view:
1. That the force exerted by any muscle during its contraction is in proportion to the number of muscular elements or fibers composing the muscle.
2. That the extent of motion, in so far as it merely depends on the shortening of the fibers of the muscle, is in proportion to the length of the fibers.
3. That the direction of the force produced by a contracting muscle is in the line of the axis of the whole muscle if it runs straight between its opposite points of attachment, but in the line of the portion attached to the moving part of the muscle, or its tendon, if it be bent in its course, etc.