The enamel is a very hard substance, the hardest in the body, and may be compared to quartz. The enamel covers the entire tooth down as far as the gums. The cement is a continuation of the enamel below the gums, and is closely adherent to the dentine. The cement consists of bone tissue, but the lamellae as a rule do not contain Haversian canals. The dentine is, next to the enamel, the hardest tissue of the tooth, and composes the main body of the tooth. The pulp cavity is found within the center of the tooth, with the opening toward the jaw bone. The tooth is nourished by a nutrient artery and vein and nerve which pass into the pulp of the tooth. =Nerves.=—Nerves are divided into two general classes, called medullary and non-medullary nerves. The non-medullated type arise mostly from the sympathetic system, while the medullated type arise from the brain and cord. As a rule, the nerves of the body follow the course of the arteries, and are generally found in the same sheath with the artery and vein. [Illustration: FIG. 6—Section of a nerve fibre. (Klein and Noble Smith)] They are easily distinguished from the arteries and veins by touch and by their color, being very inelastic and fibrous, hard to the touch, and unlike the artery or vein, since they have no central opening. =Muscles.=—Myology is that branch of anatomy which treats of the muscles. The muscles are formed of bundles of reddish fibres, endowed with the property of contractility. In the body we find two kinds of muscular tissue, called voluntary and involuntary muscle. The voluntary type is characterized by the striped appearance which it displays when seen under the microscope, and for this reason it is called striped or striated muscle. It is so named “voluntary” because it is capable of being put into action and controlled by the will. The involuntary muscles do not present any striped appearance, and consequently are called unstriped or non-striated, and are not under the control of the will. An example of voluntary muscle would be any muscle of the bony framework as for example, the biceps or triceps. [Illustration: FIG. 7—View of muscle fibers.] An example of involuntary muscle would be those of the intestines and stomach, the muscles of the bladder and uterus and the walls of the arteries and veins, etc. When viewed under the microscope, the muscle is seen to be composed of many fibrils. The sheath covering each fibril is called the sarcolemma, and contains within its boundaries the muscle plasma, or protoplasm, and a nucleus. Many of the fibrils when grouped together constitute the entire muscle. The muscles get their blood supply from the nutrient artery, which ramifies the tissues, the smallest capillaries coming in contact with each muscle cell.