The Muscles of your body either obey you such as the striped muscles, about 2.5 cm. in length attached at each at each end to a bone, producing either flexion (bending) or extension (straightening) of a limb ; or the muscles over which we have no control - the smooth or involuntary muscles of the intestines, bladder and the muscles lining the larger blood vessels. The heart muscle itself (myocardium) does the work though it is composed of a special kind of striped muscle tissue.

The Heart

The Heart is a four-chambered , double-acting pump. A septum down the middle separates it into a 'right' and 'left' heart. It has five valves, which control the flow of blood. The heart muscles is nourished by coronary arteries. Each side of the heart is subdivided into two chambers, separated by valves. The upper chambers are called 'auricles'. The lower chambers 'ventricles'. (Fig. 6.)

Blood Circulation

The circulation of blood has two courses (1)(Systemic Circulation) from left ventricle to right auricle. (2)(Pulmonary Circulation) from right ventricle to left auricle. The systemic circulation leaves by the aorta (largest artery of the body) ; this subdivided into smaller arteries, which carry blood to all parts of the body, and these finally become the fine capillaries, which in turn unite into large vessels to become the veins, which carry the blood back to the right auricle. The pulmonary (pertaining to the lungs) circulation passes the blood into the right ventricle, which pumps it through the pulmonary artery into both the lungs, whence the blood returns to the left auricle, from which it is pumped by the left ventricle into the aorta, completing the cycle. The heart normally beats from 60 to 90 times a minute, about four times the breathing (respiration) rate in a normal, healthy person. The delicately timed sequence of the heart beat (pulse) is controlled by nerve impulses from the brain and by special nerve centers within the heart. One of these nerve centers in the heart is called the pacemaker. One set of autonomic nerves speeds up the heartbeat ; another set slows it down. The wave of contraction in the beating heart (its period of systole) starts with the auricles and passes on to the ventricles. In the 'lub-dub' sound of the heart. The auricles are heard beating first. There are timed pauses in this process. The auricles are at rest when the ventricles contract, and vice versa. But there is also a time between beats (the period of diastole) when both auricles and ventricles are at rest. Infants have a much faster heartbeat than adults ; about 120 beats a minute.