Читать книгу A Practical Physiology: A Text-Book for Higher Schools - Albert F. Blaisdell - Страница 4

Table of Contents

1. The Study of Physiology. We are now to take up a new study, and in a field quite different from any we have thus far entered. Of all our other studies,--mathematics, physics, history, language,--not one comes home to us with such peculiar interest as does physiology, because this is the study of ourselves.

Every thoughtful young person must have asked himself a hundred questions about the problems of human life: how it can be that the few articles of our daily food--milk, bread, meats, and similar things--build up our complex bodies, and by what strange magic they are transformed into hair, skin, teeth, bones, muscles, and blood.

How is it that we can lift these curtains of our eyes and behold all the wonders of the world around us, then drop the lids, and though at noonday, are instantly in total darkness? How does the minute structure of the ear report to us with equal accuracy the thunder of the tempest, and the hum of the passing bee? Why is breathing so essential to our life, and why cannot we stop breathing when we try? Where within us, and how, burns the mysterious fire whose subtle heat warms us from the first breath of infancy till the last hour of life?

These and scores of similar questions it is the province of this deeply interesting study of physiology to answer.

2. What Physiology should Teach us. The study of physiology is not only interesting, but it is also extremely useful. Every reasonable person should not only wish to acquire the knowledge how best to protect and preserve his body, but should feel a certain profound respect for an organism so wonderful and so perfect as his physical frame. For our bodies are indeed not ourselves, but the frames that contain us,--the ships in which we, the real selves, are borne over the sea of life. He must be indeed a poor navigator who is not zealous to adorn and strengthen his ship, that it may escape the rocks of disease and premature decay, and that the voyage of his life may be long, pleasant, and successful.

But above these thoughts there rises another,--that in studying physiology we are tracing the myriad lines of marvelous ingenuity and forethought, as they appear at every glimpse of the work of the Divine Builder. However closely we study our bodily structure, we are, at our best, but imperfect observers of the handiwork of Him who made us as we are.

3. Distinctive Characters of Living Bodies. Even a very meagre knowledge of the structure and action of our bodies is enough to reveal the following distinctive characters: our bodies are continually breathing, that is, they take in oxygen from the surrounding air; they take in certain substances known as food, similar to those composing the body, which are capable through a process called oxidation, or through other chemical changes, of setting free a certain amount of energy.

Again, our bodies are continually making heat and giving it out to surrounding objects, the production and the loss of heat being so adjusted that the whole body is warm, that is, of a temperature higher than that of surrounding objects. Our bodies, also, move themselves, either one part on another, or the whole body from place to place. The motive power is not from the outside world, but the energy of their movements exists in the bodies themselves, influenced by changes in their surroundings. Finally, our bodies are continually getting rid of so-called waste matters, which may be considered products of the oxidation of the material used as food, or of the substances which make up the organism.

4. The Main Problems of Physiology briefly Stated. We shall learn in a subsequent chapter that the living body is continually losing energy, but by means of food is continually restoring its substance and replenishing its stock of energy. A great deal of energy thus stored up is utilized as mechanical work, the result of physical movements. We shall learn later on that much of the energy which at last leaves the body as heat, exists for a time within the organism in other forms than heat, though eventually transformed into heat. Even a slight change in the surroundings of the living body may rapidly, profoundly, and in special ways affect not only the amount, but the kind of energy set free. Thus the mere touch of a hair may lead to such a discharge of energy, that a body previously at rest may be suddenly thrown into violent convulsions. This is especially true in the case of tetanus, or lockjaw.

The main problem we have to solve in the succeeding pages is to ascertain how it is that our bodies can renew their substance and replenish the energy which they are continually losing, and can, according to the nature of their surroundings, vary not only the amount, but the kind of energy which they set free.

5. Technical Terms Defined. All living organisms are studied usually from two points of view: first, as to their form and structure; second, as to the processes which go on within them. The science which treats of all living organisms is called biology. It has naturally two divisions,--morphology, which treats of the form and structure of living beings, and physiology, which investigates their functions, or the special work done in their vital processes.

The word anatomy, however, is usually employed instead of morphology. It is derived from two Greek words, and means the science of dissection. Human anatomy then deals with the form and structure of the human body, and describes how the different parts and organs are arranged, as revealed by observation, by dissection, and by the microscope.

Histology is that part of anatomy which treats of the minute structure of any part of the body, as shown by the microscope.

Human physiology describes the various processes that go on in the human body in health. It treats of the work done by the various parts of the body, and of the results of the harmonious action of the several organs. Broadly speaking, physiology is the science which treats of functions. By the word function is meant the special work which an organ has to do. An organ is a part of the body which does a special work. Thus the eye is the organ of sight, the stomach of digestion, and the lungs of breathing.

It is plain that we cannot understand the physiology of our bodies without a knowledge of their anatomy. An engineer could not understand the working of his engine unless well acquainted with all its parts, and the manner in which they were fitted together. So, if we are to understand the principles of elementary physiology, we must master the main anatomical facts concerning the organs of the body before considering their special functions.

As a branch of study in our schools, physiology aims to make clear certain laws which are necessary to health, so that by a proper knowledge of them, and their practical application, we may hope to spend happier and more useful, because healthier, lives. In brief, the study of hygiene, or the science of health, in the school curriculum, is usually associated with that of physiology.[1]

6. Chemical Elements in the Body. All of the various complex substances found in nature can be reduced by chemical analysis to about 70 elements, which cannot be further divided. By various combinations of these 70 elements all the substances known to exist in the world of nature are built up. When the inanimate body, like any other substance, is submitted to chemical analysis, it is found that the bone, muscle, teeth, blood, etc., may be reduced to a few chemical elements.

In fact, the human body is built up with 13 of the 70 elements, namely: oxygen, hydrogen, nitrogen, chlorine, fluorine, carbon, phosphorus, sulphur, calcium, potassium, sodium, magnesium, and iron. Besides these, a few of the other elements, as silicon, have been found; but they exist in extremely minute quantities.

The following table gives the proportion in which these various elements are present:

Oxygen	62.430	per cent
Carbon	21.150	" "
Hydrogen	9.865	" "
Nitrogen	3.100	" "
Calcium	1.900	" "
Phosphorus	0.946	" "
Potassium	0.230	" "
Sulphur	0.162	" "
Chlorine	0.081	" "
Sodium	0.081	" "
Magnesium	0.027	" "
Iron	0.014	" "
Fluorine	0.014	" "
	-----
	100.000

As will be seen from this table, oxygen, hydrogen, and nitrogen, which are gases in their uncombined form, make up ¾ of the weight of the whole human body. Carbon, which exists in an impure state in charcoal, forms more than ⅕ of the weight of the body. Thus carbon and the three gases named, make up about 96 per cent of the total weight of the body.

7. Chemical Compounds in the Body. We must keep in mind that, with slight exceptions, none of these 13 elements exist in their elementary form in the animal economy. They are combined in various proportions, the results differing widely from the elements of which they consist. Oxygen and hydrogen unite to form water, and water forms more than ⅔ of the weight of the whole body. In all the fluids of the body, water acts as a solvent, and by this means alone the circulation of nutrient material is possible. All the various processes of secretion and nutrition depend on the presence of water for their activities.

8. Inorganic Salts. A large number of the elements of the body unite one with another by chemical affinity and form inorganic salts. Thus sodium and chlorine unite and form chloride of sodium, or common salt. This is found in all the tissues and fluids, and is one of the most important inorganic salts the body contains. It is absolutely necessary for continued existence. By a combination of phosphorus with sodium, potassium, calcium, and magnesium, the various phosphates are formed.

The phosphates of lime and soda are the most abundant of the salts of the body. They form more than half the material of the bones, are found in the teeth and in other solids and in the fluids of the body. The special place of iron is in the coloring matter of the blood. Its various salts are traced in the ash of bones, in muscles, and in many other tissues and fluids. These compounds, forming salts or mineral matters that exist in the body, are estimated to amount to about 6 per cent of the entire weight.

9. Organic Compounds. Besides the inorganic materials, there exists in the human body a series of compound substances formed of the union of the elements just described, but which require the agency of living structures. They are built up from the elements by plants, and are called organic. Human beings and the lower animals take the organized materials they require, and build them up in their own bodies into still more highly organized forms.

The organic compounds found in the body are usually divided into three great classes:

1 Proteids, or albuminous substances.

2 Carbohydrates (starches, sugars, and gums).

3 Fats.

The extent to which these three great classes of organic materials of the body exist in the animal and vegetable kingdoms, and are utilized for the food of man, will be discussed in the chapter on food (Chapter V.). The Proteids, because they contain the element nitrogen and the others do not, are frequently called nitrogenous, and the other two are known as non-nitrogenous substances. The proteids, the type of which is egg albumen, or the white of egg, are found in muscle and nerve, in glands, in blood, and in nearly all the fluids of the body. A human body is estimated to yield on an average about 18 per cent of albuminous substances. In the succeeding chapters we shall have occasion to refer to various and allied forms of proteids as they exist in muscle (myosin), coagulated blood (fibrin), and bones (gelatin).

The Carbohydrates are formed of carbon, hydrogen, and oxygen, the last two in the proportion to form water. Thus we have animal starch, or glycogen, stored up in the liver. Sugar, as grape sugar, is also found in the liver. The body of an average man contains about 10 per cent of Fats. These are formed of carbon, hydrogen, and oxygen, in which the latter two are not in the proportion to form water. The fat of the body consists of a mixture which is liquid at the ordinary temperature.

Now it must not for one moment be supposed that the various chemical elements, as the proteids, the salts, the fats, etc., exist in the body in a condition to be easily separated one from another. Thus a piece of muscle contains all the various organic compounds just mentioned, but they are combined, and in different cases the amount will vary. Again, fat may exist in the muscles even though it is not visible to the naked eye, and a microscope is required to show the minute fat cells.

10. Protoplasm. The ultimate elements of which the body is composed consist of "masses of living matter," microscopic in size, of a material commonly called protoplasm.[2] In its simplest form protoplasm appears to be a homogeneous, structureless material, somewhat resembling the raw white of an egg. It is a mixture of several chemical substances and differs in appearance and composition in different parts of the body.

Protoplasm has the power of appropriating nutrient material, of dividing and subdividing, so as to form new masses like itself. When not built into a tissue, it has the power of changing its shape and of moving from place to place, by means of the delicate processes which it puts forth. Now, while there are found in the lowest realm of animal life, organisms like the amœba of stagnant pools, consisting of nothing more than minute masses of protoplasm, there are others like them which possess a small central body called a nucleus. This is known as nucleated protoplasm.

Fig. 1.--Diagram of a Cell.

 A, nucleus;

 B, nucleolus;

 C, protoplasm. (Highly magnified)

11. Cells. When we carry back the analysis of an organized body as far as we can, we find every part of it made up of masses of nucleated protoplasm of various sizes and shapes. In all essential features these masses conform to the type of protoplasmic matter just described. Such bodies are called cells. In many cells the nucleus is finely granular or reticulated in appearance, and on the threads of the meshwork may be one or more enlargements, called nucleoli. In some cases the protoplasm at the circumference is so modified as to give the appearance of a limiting membrane called the cell wall. In brief, then, a cell is a mass of nucleated protoplasm; the nucleus may have a nucleolus, and the cell may be limited by a cell wall. Every tissue of the human body is formed through the agency of protoplasmic cells, although in most cases the changes they undergo are so great that little evidence remains of their existence.

There are some organisms lower down in the scale, whose whole activity is confined within the narrow limits of a single cell. Thus, the amœba begins its life as a cell split off from its parent. This divides in its turn, and each half is a complete amœba. When we come a little higher than the amœba, we find organisms which consist of several cells, and a specialization of function begins to appear. As we ascend in the animal scale, specialization of structure and of function is found continually advancing, and the various kinds of cells are grouped together into colonies or organs.

12. Cells and the Human Organism. If the body be studied in its development, it is found to originate from a single mass of nucleated protoplasm, a single cell with a nucleus and nucleolus. From this original cell, by growth and development, the body, with all its various tissues, is built up. Many fully formed organs, like the liver, consist chiefly of cells. Again, the cells are modified to form fibers, such as tendon, muscle, and nerve. Later on, we shall see the white blood corpuscles exhibit all the characters of the amœba (Fig. 2). Even such dense structures as bone, cartilage, and the teeth are formed from cells.

Fig. 2.--Amœboid Movement of a Human White Blood Corpuscle. (Showing various phases of movement.)

In short, cells may be regarded as the histological units of animal structures; by the combination, association, and modification of these the body is built up. Of the real nature of the changes going on within the living protoplasm, the process of building up lifeless material into living structures, and the process of breaking down by which waste is produced, we know absolutely nothing. Could we learn that, perhaps we should know the secret of life.

13. Kinds of Cells. Cells vary greatly in size, some of the smallest being only ⅓500 an inch or less in diameter. They also vary greatly in form, as may be seen in Figs. 3 and 5. The typical cell is usually globular in form, other shapes being the result of pressure or of similar modifying influences. The globular, as well as the large, flat cells, are well shown in a drop of saliva. Then there are the columnar cells, found in various parts of the intestines, in which they are closely arranged side by side. These cells sometimes have on the free surface delicate prolongations called cilia. Under the microscope they resemble a wave, as when the wind blows over a field of grain (Fig. 5). There are besides cells known as spindle, stellate, squamous or pavement, and various other names suggested by their shapes. Cells are also described as to their contents. Thus fat and pigment cells are alluded to in succeeding sections. Again, they may be described as to their functions or location or the tissue in which they are found, as epithelial cells, blood cells (corpuscles, Figs. 2 and 66), nerve cells (Fig. 4), and connective-tissue cells.

14. Vital Properties of Cells. Each cell has a life of its own. It manifests its vital properties in that it is born, grows, multiplies, decays, and at last dies.[3] During its life it assimilates food, works, rests, and is capable of spontaneous motion and frequently of locomotion. The cell can secrete and excrete substance, and, in brief, presents nearly all the phenomena of a human being.

Cells are produced only from cells by a process of self-division, consisting of a cleavage of the whole cell into parts, each of which becomes a separate and independent organism. Cells rapidly increase in size up to a certain definite point which they maintain during adult life. A most interesting quality of cell life is motion, a beautiful form of which is found in ciliated epithelium. Cells may move actively and passively. In the blood the cells are swept along by the current, but the white corpuscles, seem able to make their way actively through the tissues, as if guided by some sort of instinct.

Fig. 3.--Various Forms of Cells.

 A, columnar cells found lining various parts of the intestines (called columnar epithelium);

 B, cells of a fusiform or spindle shape found in the loose tissue under the skin and in other parts (called connective-tissue cells);

 C, cell having many processes or projections--such are found in connective tissue, D, primitive cells composed of protoplasm with nucleus, and having no cell wall. All are represented about 400 times their real size.

Some cells live a brief life of 12 to 24 hours, as is probably the case with many of the cells lining the alimentary canal; others may live for years, as do the cells of cartilage and bone. In fact each cell goes through the same cycle of changes as the whole organism, though doubtless in a much shorter time. The work of cells is of the most varied kind, and embraces the formation of every tissue and product,--solid, liquid, or gaseous. Thus we shall learn that the cells of the liver form bile, those of the salivary glands and of the glands of the stomach and pancreas form juices which aid in the digestion of food.

15. The Process of Life. All living structures are subject to constant decay. Life is a condition of incessant changes, dependent upon two opposite processes, repair and decay. Thus our bodies are not composed of exactly the same particles from day to day, or even from one moment to another, although to all appearance we remain the same individuals. The change is so gradual, and the renewal of that which is lost may be so exact, that no difference can be noticed except at long intervals of time.[4] (See under "Bacteria," Chapter XIV.)

The entire series of chemical changes that take place in the living body, beginning with assimilation and ending with excretion, is included in one word, metabolism. The process of building up living material, or the change by which complex substances (including the living matter itself) are built up from simpler materials, is called anabolism. The breaking down of material into simple products, or the changes in which complex materials (including the living substance) are broken down into comparatively simple products, is known as katabolism. This reduction of complex substances to simple, results in the production of animal force and energy. Thus a complex substance, like a piece of beef-steak, is built up of a large number of molecules which required the expenditure of force or energy to store up. Now when this material is reduced by the process of digestion to simpler bodies with fewer molecules, such as carbon dioxid, urea, and water, the force stored up in the meat as potential energy becomes manifest and is used as active life-force known as kinetic energy.

16. Epithelium. Cells are associated and combined in many ways to form a simple tissue. Such a simple tissue is called an epithelium or surface-limiting tissue, and the cells are known as epithelial cells. These are united by a very small amount of a cement substance which belongs to the proteid class of material. The epithelial cells, from their shape, are known as squamous, columnar, glandular, or ciliated. Again, the cells may be arranged in only a single layer, or they may be several layers deep. In the former case the epithelium is said to be simple; in the latter, stratified. No blood-vessels pass into these tissues; the cells derive their nourishment by the imbibition of the plasma of the blood exuded into the subjacent tissue.

Fig. 4.--Nerve Cells from the Gray Matter of the Cerebellum. (Magnified 260 diameters.)

17. Varieties of Epithelium. The squamous or pavement epithelium consists of very thin, flattened scales, usually with a small nucleus in the center. When the nucleus has disappeared, they become mere horny plates, easily detached. Such cells will be described as forming the outer layer of the skin, the lining of the mouth and the lower part of the nostrils.

The columnar epithelium consists of pear-shaped or elongated cells, frequently as a single layer of cells on the surface of a mucous membrane, as on the lining of the stomach and intestines, and the free surface of the windpipe and large air-tubes.

The glandular or spheroidal epithelium is composed of round cells or such as become angular by mutual pressure. This kind forms the lining of glands such as the liver, pancreas, and the glands of the skin.

The ciliated epithelium is marked by the presence of very fine hair-like processes called cilia, which develop from the free end of the cell and exhibit a rapid whip-like movement as long as the cell is alive. This motion is always in the same direction, and serves to carry away mucus and even foreign particles in contact with the membrane on which the cells are placed. This epithelium is especially common in the air passages, where it serves to keep a free passage for the entrance and exit of air. In other canals a similar office is filled by this kind of epithelium.

18. Functions of Epithelial Tissues. The epithelial structures may be divided, as to their functions, into two main divisions. One is chiefly protective in character. Thus the layers of epithelium which form the superficial layer of the skin have little beyond such an office to discharge. The same is to a certain extent true of the epithelial cells covering the mucous membrane of the mouth, and those lining the air passages and air cells of the lungs.

Fig. 5.--Various Kinds of Epithelial Cells

 A, columnar cells of intestine;

 B, polyhedral cells of the conjunctiva;

 C, ciliated conical cells of the trachea;

 D, ciliated cell of frog's mouth;

 E, inverted conical cell of trachea;

 F, squamous cell of the cavity of mouth, seen from its broad surface;

 G, squamous cell, seen edgeways.

The second great division of the epithelial tissues consists of those whose cells are formed of highly active protoplasm, and are busily engaged in some sort of secretion. Such are the cells of glands,--the cells of the salivary glands, which secrete the saliva, of the gastric glands, which secrete the gastric juice, of the intestinal glands, and the cells of the liver and sweat glands.

19. Connective Tissue. This is the material, made up of fibers and cells, which serves to unite and bind together the different organs and tissues. It forms a sort of flexible framework of the body, and so pervades every portion that if all the other tissues were removed, we should still have a complete representation of the bodily shape in every part. In general, the connective tissues proper act as packing, binding, and supporting structures. This name includes certain tissues which to all outward appearance vary greatly, but which are properly grouped together for the following reasons: first, they all act as supporting structures; second, under certain conditions one may be substituted for another; third, in some places they merge into each other.

All these tissues consist of a ground-substance, or matrix, cells, and fibers. The ground-substance is in small amount in connective tissues proper, and is obscured by a mass of fibers. It is best seen in hyaline cartilage, where it has a glossy appearance. In bone it is infiltrated with salts which give bone its hardness, and make it seem so unlike other tissues. The cells are called connective-tissue corpuscles, cartilage cells, and bone corpuscles, according to the tissues in which they occur. The fibers are the white fibrous and the yellow elastic tissues.

The following varieties are usually described:

1 Connective Tissues Proper:White Fibrous Tissue.Yellow Elastic Tissue.Areolar or Cellular Tissue.Adipose or Fatty Tissue.Adenoid or Retiform Tissue.

2 Cartilage (Gristle):Hyaline.White Fibro-cartilage.Yellow Fibro-cartilage.

3 Bone and Dentine of Teeth.

20. White Fibrous Tissue. This tissue consists of bundles of very delicate fibrils bound together by a small amount of cement substance. Between the fibrils protoplasmic masses (connective-tissue corpuscles) are found. These fibers may be found so interwoven as to form a sheet, as in the periosteum of the bone, the fasciæ around muscles, and the capsules of organs; or they may be aggregated into bundles and form rope-like bands, as in the ligaments of joints and the tendons of muscles. On boiling, this tissue yields gelatine. In general, where white fibrous tissue abounds, structures are held together, and there is flexibility, but little or no distensibility.

Fig. 6.--White Fibrous Tissue. (Highly magnified.)

21. Yellow Elastic Tissue. The fibers of yellow elastic tissue are much stronger and coarser 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 vertebræ together (Fig. 26), in the true vocal cords, and in certain ligaments of the larynx. In the skin and fasciæ, 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.

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 spaces or meshes. These little spaces, 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 quantity 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.

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-glass 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 vertebræ, 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.

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 canal 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 canal, 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.