Ordinarily a muscle has some object in contracting, such as the raising of a load, and it contracts voluntarily more or less according to the weight of the load. The amount of work done is calculated in foot-pounds or gram-meters, that is, the energy required to raise one pound one foot or one gram one meter. As a rule the muscles with the longest fibers, as the biceps, do the most work and those with a large number of fibers do more than those with less. It has been calculated that whereas an engine gives back one-twelfth of the energy of the coal consumed, muscle liberates one-fourth of the energy brought to it in the form of food. During activity the glycogen or sugar in the muscle is used up and the muscle becomes more acid, owing to the lactic acid that is formed. The carbon is taken in and carbon dioxide given off. Nitrogen puts the muscle in condition to do its work but is not so much used up in the work as is the carbohydrate material. So it is the non-nitrogenous matter that does the work and any increase in urea, the end-product of protein metabolism, is mere wear and tear.
Sudden heat or cold causes muscular contraction and moderate heat favors both muscular and nervous irritability. Moderate cold, however, lessens the force of contraction and below zero muscle very largely loses its irritability without necessarily becoming rigid.
While well supplied with blood, muscle will contract without fatigue, but if the blood supply is shut off, it soon loses its irritability and becomes rigid. The more a muscle is used in moderation the more it develops, but after it has done a certain amount of work it becomes exhausted, losing its irritability or power to respond to stimuli and later becoming rigid. Such fatigue is due to the production of certain poisonous waste products which have a paralyzing effect on the nerves and which are ordinarily gradually carried away in the blood, but which sometimes, if produced to excess, accumulate too fast for the blood wholly to remove them. Usually the nerve becomes exhausted first and the muscle substance later. So long as it is connected with the nervous system a muscle will respond to stimuli, but when the nerve becomes tired, degeneration is more rapid. In fact, the degree of exhaustion is determined by several factors, as by relation to the central nervous system, variations in temperature, blood supply, and functional activity, the process being more rapid in warm than in cold blooded animals.
Cilia.—A few motions are accomplished by tissue that is not muscular, as in the case of the cilia attached to the cells of the respiratory tract, which lie flat on the free surface and then lash forward, serving in the air cells to keep the air in motion and in the tubes to send secretions from below upward and outward and to keep out foreign bodies. Cilia are also found in the female genital tract, where they aid the passage of the ovum from the ovary to the womb. They act together, though apparently not governed by the nervous system. As in the white corpuscles of the blood, whose motion also is not muscular, the changes that take place in ciliated epithelium are probably about the same as those in muscular tissue, that is, contractile.
The Blood.—To most of the tissues just described nourishment is brought in the blood, which circulates through the body in a system of hollow tubes, the arteries and veins, whence it is distributed through the agency of the lymphatic system. There are no blood-vessels, however, in the epidermis, epithelium, nails, hair, teeth, nor in the cornea of the eye. The vessels that carry the blood from the heart are called arteries, those that return it veins. The former begin as large vessels and gradually decrease in size; the latter begin as small vessels and form larger and larger trunks as they approach the heart.
The arteries have three coats: 1. a thin, serous coat, the internal or intima; 2. a middle or muscular coat, and 3. an external coat of connective tissue. The middle coat is the thickest and is the one that prevents the walls from collapsing when cut across. Except in the cranium, each artery is enclosed in a sheath with its vein or veins, the venæ comites. Usually the arteries occupy protected situations and are straight in their course. Where a vessel has to accommodate itself to the movements of a part, however, it may be curved, as in the case of the facial artery which is curled on itself to allow for movements of the jaw. They anastomose or communicate freely with one another, thus promoting equality of distribution and pressure and making good circulation possible even after the obliteration of a large vessel.
The veins have three coats like the arteries, but they are not so thick and the muscular coat is not so highly developed, so that the walls collapse when cut and have no elasticity. There are constrictions on the surface of many of the veins due to the presence of valves. These valves are formed of semilunar folds of the lining membrane and are arranged in pairs. They serve to prevent the blood, whose circulation in the veins is sluggish, from flowing back.
There are two sets of veins, the superficial and the deep, which communicate with each other. In fact, all the veins, large and small, anastomose very freely, especially in the skull and neck, where obstruction would result in serious trouble, throughout the spinal cord, and in the abdomen and pelvis. The deep veins accompany the arteries in their sheath, while the superficial ones have thicker walls and run between the layers of the superficial fascia under the skin, terminating in the deep veins. In the skull the venous channels take the form of sinuses, formed by a separating of the layers of the dura mater, with an endothelial lining that is continuous with that of the veins.
The [capillaries] are intermediate between the arteries and the veins, the final division of the arteries and the first source of the veins. They are tiny vessels with but a single coat, continuous with the innermost coat of both arteries and veins and consisting practically of one layer of cells with a small amount of connective tissue between. They spread in a great network throughout the tissues, forming plexuses and being especially abundant where the blood is needed for other purposes than local nutrition, as in the secreting glands. Their diameter is so small that the red corpuscles have to pass in single file and may even then be squeezed out of shape. As they have no muscular tissue in their walls, they have no power of contracting. Their walls, however, like those of the smaller arteries and veins, are porous and by virtue of this quality they play an important part in the economy, since in them the exchange takes place between the tissues and the blood.
The arteries in general carry freshly oxidized blood and the veins blood from which the oxygen has been largely used up and which contains waste material. In the pulmonary system, however, the case is reversed, the pulmonary arteries conveying venous blood, as it is called, from the heart to the lungs to be oxidized and the veins returning the blood after it has received its new supply of oxygen.