21. Yellow Elastic Tissue. The fibers of yellow elastic tissue are much stronger and coa.r.s.er than those of the white. They are yellowish, tend to curl up at the ends, and are highly elastic. It is these fibers which give elasticity to the skin and to the coats of the arteries. The typical form of this tissue occurs in the ligaments which bind the vertebrae together (Fig. 26), in the true vocal cords, and in certain ligaments of the larynx. In the skin and fasciae, the yellow elastic is found mixed with white fibrous and areolar tissues. It does not yield gelatine on boiling, and the cells are, if any, few.

[Ill.u.s.tration: Fig. 7.--Yellow Elastic Tissue. (Highly magnified.)]

22. Areolar or Cellular Tissue. This consists of bundles of delicate fibers interlacing and crossing one another, forming irregular s.p.a.ces or meshes. These little s.p.a.ces, in health, are filled with fluid that has oozed out of the blood-vessels. The areolar tissue forms a protective covering for the tissues of delicate and important organs.

23. Adipose or Fatty Tissue. In almost every part of the body the ordinary areolar tissue contains a variable quant.i.ty of adipose or fatty tissue. Examined by the microscope, the fat cells consist of a number of minute sacs of exceedingly delicate, structureless membrane filled with oil. This is liquid in life, but becomes solidified after death. This tissue is plentiful beneath the skin, in the abdominal cavity, on the surface of the heart, around the kidneys, in the marrow of bones, and elsewhere. Fat serves as a soft packing material. Being a poor conductor, it retains the heat, and furnishes a store rich in carbon and hydrogen for use in the body.

24. Adenoid or Retiform Tissue. This is a variety of connective tissue found in the tonsils, spleen, lymphatic glands, and allied structures. It consists of a very fine network of cells of various sizes.

The tissue combining them is known as adenoid or gland-like tissue.

[Ill.u.s.tration: Fig. 8.--Fibro-Cartilage Fibers. (Showing network surrounded cartilage cells.)]

25. Cartilage. Cartilage, or gristle, is a tough but highly elastic substance. Under the microscope cartilage is seen to consist of a matrix, or base, in which nucleated cells abound, either singly or in groups. It has sometimes a fine ground-gla.s.s appearance, when the cartilage is spoken of as hyaline. In other cases the matrix is almost replaced by white fibrous tissue. This is called white fibro-cartilage, and is found where great strength and a certain amount of rigidity are required.

Again, there is between the cells a meshwork of yellow elastic fibers, and this is called yellow fibro-cartilage (Fig. 8). The hyaline cartilage forms the early state of most of the bones, and is also a permanent coating for the articular ends of long bones. The white fibro-cartilage is found in the disks between the bodies of the vertebrae, in the interior of the knee joint, in the wrist and other joints, filling the cavities of the bones, in socket joints, and in the grooves for tendons. The yellow fibro-cartilage forms the expanded part of the ear, the epiglottis, and other parts of the larynx.

26. General Plan of the Body. To get a clearer idea of the general plan on which the body is constructed, let us imagine its division into perfectly equal parts, one the right and the other the left, by a great knife severing it through the median, or middle line in front, backward through the spinal column, as a butcher divides an ox or a sheep into halves for the market. In a section of the body thus planned the skull and the spine together are shown to have formed a tube, containing the brain and spinal cord. The other parts of the body form a second tube (ventral) in front of the spinal or dorsal tube. The upper part of the second tube begins with the mouth and is formed by the ribs and breastbone. Below the chest in the abdomen, the walls of this tube would be made up of the soft parts.

[Ill.u.s.tration: Fig. 9.--Diagrammatic Longitudinal Section of the Trunk and Head. (Showing the dorsal and the ventral tubes.)

A, the cranial cavity; B, the cavity of the nose; C, the mouth; D, the alimentary ca.n.a.l represented as a simple straight tube; E, the sympathetic nervous system; F, heart; G, diaphragm; H, stomach; K, end of spinal portion of cerebro-spinal nervous system.

We may say, then, that the body consists of two tubes or cavities, separated by a bony wall, the dorsal or nervous tube, so called because it contains the central parts of the nervous system; and the visceral or ventral tube, as it contains the viscera, or general organs of the body, as the alimentary ca.n.a.l, the heart, the lungs, the sympathetic nervous system, and other organs.

The more detailed study of the body may now be begun by a description of the skeleton or framework which supports the soft parts.

Experiments.

For general directions and explanations and also detailed suggestions for performing experiments, see Chapter XV.

Experiment 1. _To examine squamous epithelium._ With an ivory paper-knife sc.r.a.pe the back of the tongue or the inside of the lips or cheek; place the substance thus obtained upon a gla.s.s slide; cover it with a thin cover-gla.s.s, and if necessary add a drop of water. Examine with the microscope, and the irregularly formed epithelial cells will be seen.

Experiment 2. _To examine ciliated epithelium._ Open a frog's mouth, and with the back of a knife blade gently sc.r.a.pe a little of the membrane from the roof of the mouth. Transfer to a gla.s.s slide, add a drop of salt solution, and place over it a cover-gla.s.s with a hair underneath to prevent pressure upon the cells. Examine with a microscope under a high power. The cilia move very rapidly when quite fresh, and are therefore not easily seen.

For additional experiments which pertain to the microscopic examination of the elementary tissues and to other points in practical histology, see Chapter XV.

[NOTE. Inasmuch as most of the experimental work of this chapter depends upon the use of the microscope and also necessarily a.s.sumes a knowledge of facts which are discussed later, it would be well to postpone experiments in histology until they can be more satisfactorily handled in connection with kindred topics as they are met with in the succeeding chapters.]

Chapter II.

The Bones.

27. The Skeleton. Most animals have some kind of framework to support and protect the soft and fleshy parts of their bodies. This framework consists chiefly of a large number of bones, and is called the skeleton. It is like the keel and ribs of a vessel or the frame of a house, the foundation upon which the bodies are securely built.

There are in the adult human body 200 distinct bones, of many sizes and shapes. This number does not, however, include several small bones found in the tendons of muscles and in the ear. The teeth are not usually reckoned as separate bones, being a part of the structure of the skin.

The number of distinct bones varies at different periods of life. It is greater in childhood than in adults, for many bones which are then separate, to allow growth, afterwards become gradually united. In early adult life, for instance, the skull contains 22 naturally separate bones, but in infancy the number is much greater, and in old age far less.

The bones of the body thus arranged give firmness, strength, and protection to the soft tissues and vital organs, and also form levers for the muscles to act upon.

28. Chemical Composition of Bone. The bones, thus forming the framework of the body, are hard, tough, and elastic. They are twice as strong as oak; one cubic inch of compact bone will support a weight of 5000 pounds. Bone is composed of earthy or mineral matter (chiefly in the form of lime salts), and of animal matter (princ.i.p.ally gelatine), in the proportion of two-thirds of the former to one-third of the latter.

[Ill.u.s.tration: Fig. 10.--The Skeleton.]

The proportion of earthy to animal matter varies with age. In infancy the bones are composed almost wholly of animal matter. Hence, an infant's bones are rarely broken, but its legs may soon become misshapen if walking is allowed too early. In childhood, the bones still contain a larger percentage of animal matter than in more advanced life, and are therefore more liable to bend than to break; while in old age, they contain a greater percentage of mineral matter, and are brittle and easily broken.

Experiment 3. _To show the mineral matter in bone_. Weigh a large soup bone; put it on a hot, clear fire until it is at a red heat. At first it becomes black from the carbon of its organic matter, but at last it turns white. Let it cool and weigh again. The animal matter has been burnt out, leaving the mineral or earthy part, a white, brittle substance of exactly the same shape, but weighing only about two-thirds as much as the bone originally weighed.

Experiment 4. _To show the animal matter in bone_. Add a teaspoonful of muriatic acid to a pint of water, and place the mixture in a shallow earthen dish. Sc.r.a.pe and clean a chicken's leg bone, part of a sheep's rib, or any other small, thin bone. Soak the bone in the acid mixture for a few days. The earthy or mineral matter is slowly dissolved, and the bone, although retaining its original form, loses its rigidity, and becomes pliable, and so soft as to be readily cut. If the experiment be carefully performed, a long, thin bone may even be tied into a knot.

[Ill.u.s.tration: Fig. 11.--The fibula tied into a knot, after the hard mineral matter has been dissolved by acid.]

29. Physical Properties of Bone. If we take a leg bone of a sheep, or a large end of beef shin bone, and saw it lengthwise in halves, we see two distinct structures. There is a hard and compact tissue, like ivory, forming the outside sh.e.l.l, and a spongy tissue inside having the appearance of a beautiful lattice work. Hence this is called cancellous tissue, and the gradual transition from one to the other is apparent.

It will also be seen that the shaft is a hollow cylinder, formed of compact tissue, enclosing a cavity called the medullary ca.n.a.l, which is filled with a pulpy, yellow fat called _marrow_. The marrow is richly supplied with blood-vessels, which enter the cavity through small openings in the compact tissue. In fact, all over the surface of bone are minute ca.n.a.ls leading into the substance. One of these, especially constant and large in many bones, is called the _nutrient foramen_, and transmits an artery to nourish the bone.

At the ends of a long bone, where it expands, there is no medullary ca.n.a.l, and the bony tissue is spongy, with only a thin layer of dense bone around it. In flat bones we find two layers or plates of compact tissue at the surface, and a spongy tissue between. Short and irregular bones have no medullary ca.n.a.l, only a thin sh.e.l.l of dense bone filled with cancellous tissue.

[Ill.u.s.tration: Fig 12.--The Right femur sawed in two, lengthwise. (Showing arrangement of compact and cancellous tissue.)]

Experiment 5. Obtain a part of a beef shin bone, or a portion of a sheep's or calf's leg, including if convenient the knee joint. Have the bone sawed in two, lengthwise, keeping the marrow in place. Boil, sc.r.a.pe, and carefully clean one half. Note the compact and spongy parts, shaft, etc.

Experiment 6. Trim off the flesh from the second half. Note the pinkish white appearance of the bone, the marrow, and the tiny specks of blood, etc. Knead a small piece of the marrow in the palm; note the oily appearance. Convert some marrow into a liquid by heating. Contrast this fresh bone with an old dry one, as found in the fields. Fresh bones should be kept in a cool place, carefully wrapped in a damp cloth, while waiting for cla.s.s use.

A fresh or living bone is covered with a delicate, tough, fibrous membrane, called the periosteum. It adheres very closely to the bone, and covers every part except at the joints and where it is protected with cartilage. The periosteum is richly supplied with blood-vessels, and plays a chief part in the growth, formation, and repair of bone. If a portion of the periosteum be detached by injury or disease, there is risk that a layer of the subjacent bone will lose its vitality and be cast off.[5]

30. Microscopic Structure of Bone. If a very thin slice of bone be cut from the compact tissue and examined under a microscope, numerous minute openings are seen. Around these are arranged rings of bone, with little black bodies in them, from which radiate fine, dark lines. These openings are sections of ca.n.a.ls called _Haversian ca.n.a.ls_, after Havers, an English physician, who first discovered them. The black bodies are minute cavities called _lacunae_, while the fine lines are very minute ca.n.a.ls, _ca.n.a.liculi_, which connect the lacunae and the Haversian ca.n.a.ls.

These Haversian ca.n.a.ls are supplied with tiny blood-vessels, while the lacunae contain bone cells. Very fine branches from these cells pa.s.s into the ca.n.a.liculi. The Haversian ca.n.a.ls run lengthwise of the bone; hence if the bone be divided longitudinally these ca.n.a.ls will be opened along their length (Fig. 13).

Thus bones are not dry, lifeless substances, but are the very type of activity and change. In life they are richly supplied with blood from the nutrient artery and from the periosteum, by an endless network of nourishing ca.n.a.ls throughout their whole structure. Bone has, therefore, like all other living structures, a _self-formative_ power, and draws from the blood the materials for its own nutrition.

[Ill.u.s.tration: Fig. 13.

A, longitudinal section of bone, by which the Haversian ca.n.a.ls are seen branching and communicating with one another; B, cross section of a very thin slice of bone, magnified about 300 diameters--little openings (Haversian ca.n.a.ls) are seen, and around them are ranged rings of bones with little black bodies (lacunae), from which branch out fine dark lines (ca.n.a.liculi); C, a bone cell, highly magnified, lying in lacuna.

The Bones of the Head.