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1. Structural Characters of Organisms.—The minute structure of living beings as shown by the microscope no doubt helps to distinguish the textures of organisms from inorganic structures. Although organic textures are found to differ very widely in their characters, they are all related in one respect, namely, that at the earliest period of their existence they consist of a minute mass of a substance called Protoplasm, known as a cell. In plants a cellular structure remains obvious in all parts of the adult, no matter how much the texture may be modified by adaptation to the requirements of any given duty or function. If we examine with the microscope the leaves, bark, wood, or pith of a plant, in all of them a cellular structure can be recognized. In the less developed members of the animal kingdom, and during the initial stages in the existence of the highest animals, the textures are composed exclusively of aggregations of living cell elements. We shall shortly see that in the more fully developed condition of the higher animals, the cells become variously modified in form and function, and the protoplasm manufactures various structures adapted to the performance of the diverse functions of the different parts. In all organic textures which can be said to be living, cells are dispersed in greater or less number, and regulate their nutrition and repair.

2. Chemical Composition.—There are no characters in the chemical composition of the textures of organic beings which can be said to be absolutely distinctive or to separate them from inorganic matter. No doubt their chemical construction frequently exhibits certain peculiarities, not seen in dead matter, which may be taken as characteristic, but living textures only differ in the general plan of arrangement and composition from that most commonly met with in the construction of inorganic materials.

In the first place, the great majority of the chemical elements which we know of, take no share in the formation of living creatures, and are never found to enter into their composition. Practically, only fifteen of some seventy elements known to chemists take part in making up the tissues of animals. The majority of these are only present in very small quantity and with no

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great constancy. On the other hand, there are four elements, namely, carbon, oxygen, hydrogen and nitrogen, which are found with such great regularity, and in so great quantity, that they may be said to make up the great bulk (97 per cent.) of the animal frame. The great constancy with which the first three of these elements occur must be regarded as a most important character of organic tissues.

Secondly, in organic substances the chemical elements are associated in much more complex and irregular proportions. Generally, a large number of atoms, of each element, are grouped together to form the molecule, and often the compound is so complex that its chemical formula remains a matter of doubt. As an example of the complexity of bodies found in organic analysis, a remarkable substance, called lecithin, which appears in the analysis of protoplasm and many tissues, may be mentioned. Its formula may be expressed thus :

FC H350,
C,H
0 - PO

JOHN (CH3),
10-C,H-OH.

{o It is peculiar in containing nitrogen and phosphorus, and in construction is said to be like a fat.

In inorganic substances, on the other hand, the elements are found to be combined, as a general rule, in simple and regular proportions. The molecules are made up of but few elements arranged in a definite manner and firmly bound together, so that they are not prone to undergo decomposition. As an example, we may take water, which has the well-known formula,

H,O. Though these bodies may be taken as types of organic and inorganic substances respectively, it must not be imagined that all organic bodies are as complex, irregular and unstable as lecithin, or that inorganic compounds, as a rule, are invariably simple and stable like water.

It is further remarkable that Carbon-an element which is exceptional in forming but few associations in the mineral world, where it chiefly combines with oxygen to form C0,-is almost

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invariably present in living textures, in which it is combined with hydrogen and nitrogen as well as oxygen in various proportions. The constancy of carbon as an ingredient of organic bodies is so great that what formerly was called organic chemistry is now often called the chemistry of the carbon compounds.

These complex associations of many atoms of carbon with many atoms of other elements, are readily dissociated when exposed to the air under even slightly disturbing influences. When heated to a certain degree they burn, i. e., unite rapidly with the oxygen of the air: and in the presence of minute organisms they putrefy. Thus instability is a general feature commonly met with in most substances of organic origin.

Chemical instability reaches the highest pitch in tissues which are actually alive and engaged in vital processes. So long as any texture lives, i. e., is capable of performing its functions, it must constantly undergo certain chemical changes, a kind of decomposition, tending to produce disintegration, and a reintegration by means of new chemical associations with fresh materials. A tissue may then be said to deserve the term living, only as long as it undergoes these antagonistic chemical changes. The tendency to destructive oxidation or disintegration is intimately connected with the functional activity of the living texture and increases with this activity. The reintegration or constructive process requires the presence of suitable materials with which the texture may combine, in order to make up for the loss. Thus living tissues are ever on the point of destruction, which can only be warded off by the timely reconstruction of their chemical ingredients by suitable fresh materials. This reconstruction by means of fresh matter from without is called assimilation, and forms the most, if not the only, satisfactory criterion by which adequately to distinguish living beings from inorganic matters.

The object of assimilation is to supply suitable fresh materials to the various textures for the chemical processes required for their function while living. This will be found to form a great part of physiological study. Further, the energy manifested in the living activity of the textures depends upon the various oxidizing processes, and the exact laws which govern these combus

tions, and the results they produce in the various tissues, practically make up the other part of physiology.

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3. Vital Phenomena.--The so-called vital phenomena which take place in the textures of organisms are, for the most part, performed by the agency of the living cell elements, in which we can recognize independent manifestations of life, such as the response to stimuli, motion, nutrition, growth, etc. The living activity of organisms requires for its perfect development certain external conditions, namely, a certain degree of warmth and moisture. Without a certain degree of warmth and moisture the chemical interchanges just mentioned cannot go on, and the organism is either destroyed or remains in a state of inactivity.

The nutrition of the animal body which is accomplished by means of the processes of assimilation already mentioned enables it to grow, and, up to a certain point, increase in size, and further undergo many changes in form and texture. There is, however, a limit to this assimilative power : nutritive activity diminishes, growth gradually stops, and after a time decay appears and is followed by death.

Thus organisms exist only for a limited period of time, during which their size, form and functional activity are constantly undergoing some general alteration dependent on or concurrent with the incessant changes in their molecular construction.

This cycle of changes through which organisms pass we speak of as their lifetime. During this lifetime, at the period when their functional activity is at its height, they possess the remarkable faculty of producing individuals like themselves.

This is accomplished by setting apart a cell which, under favorable circumstances, assumes special powers of growth, increases in size by the rapid formation of new cells, and develops into an independent living unit. In time it arrives at maturity, and becomes like its parent, and then passes through the same cycle-by its power of assimilation it grows to maturity, reproduces its like, decays and dies.

CHAPTER II.

GENERAL VIEW OF THE STRUCTURAL CHARACTERS

OF ANIMAL ORGANISMS.

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The parts played by Cells in the functions of living beings are so many and so important that it is necessary at the very outset to consider the properties of the individual elements somewhat in

3 detail.

1 The demonstration of the cellular structure of plants was first made in 1832 by a distinguished German botanist named Schlieden,

S who considered the cells to be w characteristic of plant tissue. A few years later Schwann showed that the animal tissues, though not

2 so obviously, were also made up of cells, and that they owed their beginning and development to the activity of cell elements. Thus originated the “ cellular theory," which, with some modifications, is now the basis of all physiological inquiry.

The first idea which was conveyed by the term cell varied much from that which we now accept as a proper definition of such an organic Cells from the root of a plant. (X 550.)

1. Showing youngest cells with thin unit.

walls (w), filled with protoplasm and

containing nucleus (N), and nucleo. Fully developed vegetable cells lus (N").

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2. Older cells with thicker walls with being the first discovered were taken vacuoles and cell sap (s). as the type of all. The main char- 3. Shows further diminution of proto

plasm and increase in cavity (s) in acteristics of these may be briefly

proportion to the growth of the cell wall (w),

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