This elemental living matter is called protoplasm. Its simplest form is termed a cell, and the word is applied to little bodies varying much in form. Thus some cells are really little bags filled with fluid. Such are those in which the fat is deposited; others are disk like, others lengthened, while some so-called cells are simply masses of jelly.
Out of these organic compounds all the softer tissue of the body is formed, such as fibrous, muscular, cellular, and adipose tissue.
Fibrous tissue consists of fine threads, arranged in various ways, to adapt it to some special use; some of these threads are elastic, others are wholly inelastic. These tissues of mixed character form what is called connective tissue, which is found throughout the body.
Fat is deposited in little cells situated in the connective tissue.
Muscular tissue, which forms the great bulk of the body, is easily recognized by its ruddy color, and it constitutes what is known as “flesh.” It is fibrous, and if these fibers be examined under the microscope there will appear, in those under the influence of the will, transverse markings, while the muscles not under the will-power lack these markings.
Nervous tissue is that which superintends all the actions of the living body. It is accumulated in nerve centers of which the principal is the brain. It consists of minute cells in these nerve centers, and of delicate tubes filled with nervous matter throughout the organs of the body. Such tubes are called nerves.
The skeleton is the frame-work of the body, and serves to support all the softer organs as well as to protect them. It is composed of bones which serve as attachments for the muscles.
Bone is the firm tissue of the body, and to a certain degree is hard and brittle. If bone be burned its brittleness is very much increased, but if it be placed in acid it loses its brittle properties. Thus we see that bone is made of two entirely different materials. The one called “animal matter” is easily burned out, while the other called “mineral matter” resists the action of heat, but is quickly dissolved by acid. The mineral matter is a compound of lime. This it will be seen gives to the bones their rigidity. Bones are of various shapes and sizes, adapted to the work which they have to perform.
The most remarkable part of the skeleton is the spinal column, commonly called the back-bone. It is made up of twenty-six bones, or spines, which are united together in a marvelous manner, combining strength and freedom of motion. Each spine has a central aperture which communicates with that of the adjoining spines, thus forming a long canal in which the spinal cord lies. This cord connects the brain with the various parts of the body. There is a soft cushion of cartilage between each spine, which adds much to the elasticity of the back-bone, protecting the brain from shocks. This protection is further accomplished by the curvature of the spine. These bones are not placed directly over each other, but are so arranged as to give three curves to the spinal column. Along the column there are openings, at each joint, through which the nerves come to supply fibers to the different organs of the body.
The spine rests on the pelvis, which is a large double bone, basin-shaped, that sustains the abdominal organs. The pelvis, in turn, rests on the thigh bones.