The World Around Us is divisible into the organic and inorganics worlds; the organic world including all bodies which either are or have been alive, and the inorganic comprising all others.

Physiology is the study of the healthy operations which take place in living beings; and when the word is used without qualification, it is customary to consider that special reference to the physiology of the human body is intended : still, in its widest signification, it refers to all living beings, both animal and vegetable.

It is a science which goes hand in hand with Anatomy, the study of the structure of living beings; for, as is the case with an artificial mechanism, so also with the body, an acquaintance with its structure is required to explain the way in which it works.

Anatomy and physiology are not, however, co-extensive. On the one hand, there is much physiology which has little apparent connection with anatomy; and, on the other, in the present state of science, there is much anatomy which can be studied without special reference to physiology. In fact, when the anatomist rises above the mere description of the particular objects before him, he examines structures from two points of view, one of which is the physiological, and has regard to their fitness to servo some purpose useful to the being to which they belong, while the other is called the morphological view, and looks to the structural affinities of parts in the same or in different species; for example, the relations of the human limbs one to the other and to those of other animals.

There is one department of observation in which the studies of the anatomist and physiologist become identical, namely, Development; in it series of forms are met with, important as such to the anatomist, even in a strictly morphological respect, while by the physiologist they are viewed as phenomena of action of the most remarkable kind, peculiar to living beings.

When physiological investigation diverges from anatomy, it comes into close connection with other branches of science. For not only have living bodies a structure, but they consist of components subject to the laws which govern matter in the inorganic world. Thus the body consists of chemical constituents, and many of the processes taking place within it are of a chemical nature. Its materials are also subject to the ordinary laws of physics: scattered through it are varieties of mechanical appliances; special parts are set aside for optical and acoustic purposes; and others exhibit electrical phenomena of a very remarkable description. The study of physiology is therefore very dependent on both chemistry and physics. Its connection, however, with these subjects is of a different nature from its connection with anatomy; for anatomy and physiology are two closely associated departments of Biology, or the science of life; while the bond which joins biology to chemistry and physics is simply this, that living bodies, being composed of matter, are subject to the laws of matter, besides exhibiting additional laws peculiar to themselves and termed vital.

In the following pages attention will be principally directed to human physiology, but occasional reference will be made to peculiarities of function in other animals; and while matters which are peculiar to man will be pointed out, it will become apparent that all the larger facts of function, as well as structure, are common to man and other animals. Indeed, our knowledge of human physiology is largely dependent on experiments on dogs, rabbits, horses, birds, and even frogs. It will also be our business to enter into various anatomical details, to give the student a knowledge of the structures principally implicated in the physiological processes to be explained; and, in particular, it will be necessary to describe the textures or tissues of the different parts, which in great measure require the aid of the microscope for their examination. This is the department of anatomy termed Histology.

2. Living bodies are termed Organisms, because they are composed of organs, or parts devoted to different purposes; and the purpose to which any organ is devoted is called its function.

Organs are of various degrees of complexity. In organisms of the higher or more complicated description, bodies comparable with organisms of a very simple or rudimentary kind exist as textural elements. Such textural elements enter into the formation of more complex textural organs (e.g., arteries in animals, and vascular bundles in plants), which are distributed as component parts of a variety of special organs, such as eye, ear, liver, brain, etc., often the only parts alluded to when the term special organ is used.

3. All organisms are in great part composed, particularly their more active portions, of chemical combinations of a complex kind, called organic matter, together with which there are always, in addition, various mineral constituents and water entering into their composition. The most distinctive character of organic matter is that it is combustible, becoming black when heated over a flame; and, as this blackness is due to carbon, it disappears on further exposure to heat and air, leaving the ash or non-volatile mineral constituents which are always associated with organic matter.

Organic matter is divisible into two groups of substances, which are distinguished as nitrogenous, and non-nitrogenous or carbonaceous; the first containing carbon, hydrogen, oxygen, and nitrogen, and the second having no nitrogen in their constitution. The products of the complete combustion of carbonaceous matters are carbonic acid and water, while nitrogenous substances yield ammonia in addition.

The attraction of both carbon and hydrogen for oxygen is very great. Carbonic acid, consisting of one equivalent or combining proportion of carbon and two of oxygen, is the compound which is formed when carbon is freely exposed to oxygen at a high temperature; and, when oxygen and hydrogen gases are mixed, and a light applied to them, they combine with explosion, producing water, which consists of two equivalents of hydrogen and one of oxygen. Ammonia consists of one of nitrogen and three of hydrogen; and, in the complete combustion of organic matter, this hydrogen may be obtained partly from the organic matter itself, and partly from water, the oxygen of which is used in the formation of carbonic acid. In less perfect combustion, cyanogen in a state of combination may be evolved instead of ammonia, by the nitrogen of the organic matter combining with part of the carbon, in the proportion of one equivalent of carbon to one of nitrogen.

The combustibility of organic matter depends on its contained oxygen being less than sufficient to combine with its carbon and hydrogen to form carbonic acid and water, and on the complexity of its molecules. While substances found native in the inorganic world consist of elements grouped in pairs, in which the number of equivalents of the one substance bears a simple proportion to the equivalents of the other, organic substances present groups of three, four, or more elements gathered together in common union, with many equivalents of each combined in one molecule, often in proportions by no means simple; and more especially are the molecules of the nitrogenous constituents of the textures complex.

The oxidation of organic matters may take place by other means besides a burning heat. Thus it occurs in the form of putrefaction at much lower temperatures, especially when aided by abundant moisture. So also, oxidation of organic matter and the resolution thereof either into carbonic acid, water, and ammonia, or into products of less complete decomposition, take place in the interior of organisms during life, and are sometimes alluded to under the name of combustion.