*To suggest the Double Work of Glands.*-Prepare a simple filter by fitting a piece of porous paper into a gla.s.s funnel. Through this pa.s.s pure water and also water having salt dissolved in it and containing some sediment, as sand. The water and the dissolved salt pa.s.s through, while the sediment remains on the filter. Now subst.i.tute a fresh piece of paper in the funnel and drop on its surface a little solid coloring matter, such as cochineal.

Again pa.s.s the liquid through the funnel. This time it comes through colored, the color being added by the filter. Compare the filter and materials filtered to the gland and the materials concerned in secretion (blood, the liquid secreted, substances added by the gland, etc.).

[Fig. 92]

Fig. 92-*The physiological scheme.* Diagram suggesting the essential relation of the bodily activities. See Summary of Part I, page 215, and Summary of Part II, page 413.

SUMMARY OF PART I

The body is an organization of different kinds of cells; it grows through the growth and reproduction of these cells; and its life as a whole is maintained by providing such conditions as will enable the cells to keep alive. Of chief importance in the work of the body is a nutrient fluid which supplies the cells with food and oxygen and relieves them of waste.

A moving portion of this fluid, called the blood, serves as a transporting agent, while another portion, called the lymph, pa.s.ses the materials between the blood and the cells. Through their effects upon the blood and the lymph, the organs of circulation, respiration, digestion, and excretion minister in different ways to the cells, and aid in the maintenance of life. By their combined action two distinct movements are kept up in the body, as follows:

1. An _inward_ movement which carries materials from the outside of the body toward the cells.

2. An _outward_ movement which carries materials from the cells to the outside of the body.

Pa.s.sing _inward_ are the oxygen and food materials _in a condition to unite with each other_ and thereby change their potential into kinetic energy. Pa.s.sing _outward_ are the oxygen and the elements that formed the food materials _after having united_ at the cells and liberated their energy.

As a final and all-important result, there is kept up a _continuous series of chemical changes_ in the cells. These liberate the energy, provide special substances needed by the cells, and preserve the life of the body (Fig. 92).

In the chapters which follow, we are to consider the problem of adjusting the body to and of bringing it into proper relations with its surroundings.

PART II: MOTION, COORDINATION, AND SENSATION

CHAPTER XIV - THE SKELETON

One necessary means of establishing proper relations between the body and its surroundings is _motion_.(77) 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.(78) 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 per cent 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. (See Practical Work.) 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.

[Fig. 93]

Fig. 93-*Section of a long bone* (_tibia_), showing the gross structure.

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. (See Practical Work.) 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 (Fig. 93).

The arrangement of the compact and spongy substance varies with the different bones. In the short bones (wrist and ankle bones, vertebrae, 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 s.p.a.ce being filled with the spongy substance. The _red marrow_, relations of which to the red corpuscles of the blood have already been noted (page 27), occupies the minute s.p.a.ces in the spongy substance.

[Fig. 94]

Fig. 94-*Cross section of bone showing minute structure.* Magnified. 1.

Surface layer of bone. 2. Deeper portion. 3. Haversian ca.n.a.ls from which pa.s.s the ca.n.a.liculi. 4. A lacuna. Observe arrangement of lacunae at surface and in deeper portion.

*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 ca.n.a.ls and the ca.n.a.liculi (Fig. 94). These serve the general purpose of distributing nourishment through the bone. The _Haversian ca.n.a.ls_ are larger than the ca.n.a.liculi and contain small nerves and blood vessels, chiefly capillaries (Fig. 95). They extend lengthwise through the bone. The _ca.n.a.liculi_ are channels for conveying lymph. They pa.s.s out from the Haversian ca.n.a.ls 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 ca.n.a.liculi are in communication with the periosteum.

[Fig. 95]

Fig. 95-*Section showing Haversian ca.n.a.l and contents*, highly magnified (after Schafer). 1. Arterial capillary. 2. Venous capillary. 3. Nerve fibers. 4. Lymph vessel.

*The Bone Cells.*-Surrounding the Haversian ca.n.a.ls are thin layers of bone substance called the _laminae_, and within these are great numbers of irregular bodies, known as the _lacunae_. The walls of the lacunae are hard and dense, but within each is an open s.p.a.ce. In this lies a flattened body, having a nucleus, which is recognized as the _bone cell_, or the bone corpuscle (Fig. 96). 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 ca.n.a.liculi connect with the lacunae in all parts of the bone, causing them to appear under the microscope like so many burs fastened together by their projecting spines (Fig. 94).

[Fig. 96]

Fig. 96-*Bone cell* removed from the lacuna and very highly magnified.

(From Quain's _Anatomy_.)

*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 pa.s.ses through the ca.n.a.liculi 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.(79) It gets to them through the short ca.n.a.liculi 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 ca.n.a.ls. Lymph from these vessels is conveyed to the cells through the ca.n.a.liculi that connect with the Haversian ca.n.a.ls.

*Plan and Purpose of the Skeleton.*-The framework of the body is such as to adapt it to a _movable_ structure. Obviously the different parts of the body cannot be secured to a foundation, as are those of a stationary building, but must be arranged after a plan that is conducive to motion. A moving structure, as a wagon or a bicycle, has within it some strong central part to which the remainder is joined. The same is true of the skeleton. That part to which the others are attached is a long, bony axis, known as the _spinal column_. Certain parts, as the ribs and the skull, are attached directly to the spinal column, while others are attached indirectly to it. The arrangement of all the parts is such that the spinal column is made the central, cohering portion of the skeleton and also of the whole body.

Besides the general arrangement of the parts of the skeleton, there is such a grouping of the bones in each of its main divisions as will enable them to serve definite purposes. In most places they form mechanical devices for supplying special movements, and in certain places they provide for the support or protection of important organs. In most cases there is a definite combination of different bones, forming what is called the bone group.