The Skeleton : Part 1

Physiology and Hygiene for Secondary Schools
by Francis M. Walters

(Page 14 of 28)

One necessary means of establishing proper relations between the body and its surroundings is motion. Not only can the body move itself from place to place, but it is able to move surrounding objects as well. In the production of motion three important systems are employed - the muscular system, the nervous system, and a system of mechanical devices which are found mainly in the skeleton. The muscular system supplies the energy for operating the mechanical devices, while the nervous system controls the movements. Although the skeleton serves other purposes, such as giving shape to the body and protecting certain organs, its main use is that of an aid in the production of motion.

Skeleton Tissues. - The tissues employed in the construction of the skeleton are the osseous, the cartilaginous, and the connective tissues. These are known as the supporting tissues of the body. They form the bones, supply the elastic pads at the ends of the bones, and furnish strong bands, called ligaments, for fastening the bones together. The skeleton forms about 16 percent of the weight of the body. Its tissues, being of a more durable nature than the rest of the body, do not so readily decay. Especially is this true of the osseous tissue, which may be preserved indefinitely, after removal from the body, by simply keeping it dry.

The Bones. - The separate units, or parts, of which the skeleton is constructed are called bones. They are the hard structures that can be felt in all parts of the body, and they comprise nearly the entire amount of material found in the prepared skeleton. As usually estimated, the bones are 208 in number. They vary greatly in size and shape in different parts of the body.

Composition and Properties of Bones. - The most noticeable and important properties of the bones are those of hardness, stiffness, and toughness. Upon these properties the uses of the bones depend. These properties may, in turn, be shown to depend upon the presence in osseous tissue of two essentially different kinds of substance, known as the animal matter and the mineral matter. If a bone is soaked in an acid, the mineral matter is dissolved out, and as a result it loses its properties of hardness and stiffness. This is because the mineral matter supplies these properties, being composed of substances which are hard and closely resemble certain kinds of rock. The chief materials forming the mineral matter are calcium phosphate and calcium carbonate.

On the other hand, burning a bone destroys the animal matter. When this is done the bone loses its toughness, and becomes quite brittle. The property of toughness is, therefore, supplied by the animal matter. This consists mainly of a substance called ossein, which may be dissolved out of the bones by boiling them. Separated from the bones it is known as gelatine. The blood vessels and nerves in the bones, and the protoplasm of the bone cells, are also counted in with the animal matter.

If a dry bone from a full-grown, but not old, animal be weighed before and after being burned, it is found to lose about one third of its weight. From this we may conclude that about one third of the bone by weight is animal matter and two thirds is mineral matter. This proportion, however, varies with age, the mineral matter increasing with advance of years.

Gross Structure of Bones. - The gross structure of the bones is best learned by studying both dry and fresh specimens. The ends of the bones are capped by a layer of smooth, elastic cartilage, while all the remaining surface is covered by a rather dense sheath of connective tissue, called the periosteum. Usually the central part of the long bones is hollow, being filled with a fatty substance known as the yellow marrow. Around the marrow cavity the bone is very dense and compact, but most of the material forming the ends is porous and spongy. These materials are usually referred to as the compact substance and the cancellous, or spongy, substance of the bones.

The arrangement of the compact and spongy substance varies with the different bones. In the short bones (wrist and ankle bones, vertebra, etc.) and also in the flat bones (skull bones, ribs, shoulder blades, etc.) there is no cavity for the yellow marrow, all of the interior space being filled with the spongy substance. The red marrow, relations of which to the red corpuscles of the blood have already been noted, occupies the minute spaces in the spongy substance.

Minute Structure of Bone. - A microscopic examination of a thin slice of bone taken from the compact substance shows this to be porous as well as the spongy substance. Two kinds of small channels are found running through it in different directions, known as the Haversian canals and the canaliculus. These serve the general purpose of distributing nourishment through the bone. The Haversian canals are larger than the canaliculus and contain small nerves and blood vessels, chiefly capillaries. They extend lengthwise through the bone. The canaliculus are channels for conveying lymph. They pass out from the Haversian canals at right angles, going to all portions of the compact substance except a thin layer at the surface. In the surface layer of the bone the canaliculus are in communication with the periosteum.

The Bone Cells. - Surrounding the Haversian canals are thin layers of bone substance called the lamine, and within these are great numbers of irregular bodies, known as the lacune. The walls of the lacune are hard and dense, but within each is an open space. In this lies a flattened body, having a nucleus, which is recognized as the bone cell, or the bone corpuscle. It appears to be the work of the bone cells to deposit mineral matter in the walls surrounding them and in this way to supply the properties of hardness and stiffness to the bones. The canaliculus connect with the lacune in all parts of the bone, causing them to appear under the microscope like so many burs fastened together by their projecting spines.

How the Bone Cells are Nourished. - The bone cells, like all the other cells of the body, are nourished by the lymph that escapes from the blood. This passes through the canaliculus to the cells in the different parts of the bone, as follows:

1. The cells in the surface layer of the bone receive lymph from the capillaries in the periosteum. It gets to them through the short canaliculus that run out to the surface.

2. The cells within the interior of the bone receive their nourishment from the small blood vessels in the Haversian canals. Lymph from these vessels is conveyed to the cells through the canaliculus that connect with the Haversian canals.

D.C. Heath and Co. - Publishers
Original copyright 1909