Physiology is a science, because it embodies a collection of general principles and ascertained truths relating to a particular subject, and is called a natural science because these truths are founded on observation. The word “physiology” is derived from two Greek words meaning a discourse about nature; but it is used in a restricted sense, and is the science of the functions of the different parts of any living body. Thus we have animal and vegetable physiology, while the former is divided into human and comparative. The first of these divisions relates only to man, while the other reviews the entire animal kingdom.
Our object is to teach some of the simple truths of human physiology; such as may be intelligible without any extended knowledge of other sciences. It must be remembered, however, that a more thorough and complete study of physiology can not be undertaken without a considerable acquaintance with such sciences as mechanics, hydraulics, optics, etc., without which the action of the muscles, the circulation of fluids, and vision can not be properly and fully comprehended.
Whenever a piece of mechanism, designed for some particular use, is brought under our notice, and we wish to understand its manner of working, we naturally inquire about its structure; for without some knowledge of how its component parts are put together, and by what means it is put in motion, we can not hope to understand how it performs the part which we see it do. Such is the case, for example, with a watch or steam engine; their parts must be carefully studied in order that their workings may be fully understood.
Hence, it will be observed from what has been said, that it is impossible to study the uses of various parts of the body without some knowledge of anatomy, this being that branch of knowledge which treats of structure.
In order that anatomy may be studied the organs must be dissected.
The words “anatomy” and “dissection” have the same literal meaning, the former being derived from the Greek language, and the latter from the Latin, meaning to cut apart or separate. But anatomy is employed to signify the science of structure in living bodies, and dissection is used to denote the unravelling or laying bare the parts of the body, by means of which anatomy is studied.
The words “organ,” “organization,” and “organize,” are so convenient and necessary that we must know what they mean. The word “organ” signifies some part or parts of the body that have a particular use or function: thus, muscles are organs of motion, and nerves of sensation; the eye is the organ of sight, and the heart and blood vessels are the organs for circulating the blood. Now, any structure is said to be organized, or to have organization, that possesses the properties which distinguish a living body from one that never had life. Therefore we speak of the organic world as distinguished from the inorganic; the former includes plants and animals, the latter minerals, etc.
The nature of that mysterious principle which we call life is unknown to scientists. Yet we may know and understand many of those things which are believed by physiologists concerning life. Certain of them believe that life is a phenomenon that follows organization; or, in other words, that organization is the cause, life the result; while others contend that organization is the result of life. In the former life is produced by changes that take place in matter, under the influence of those forces of nature called heat, light, electricity, and chemism; by the latter theory all these forces are present and act under the influence of a potent force called life. This will be our belief: Life is a distinct endowment, capable of propagation, and superior to all other forces by which it is attended. Let us see by what means living bodies are distinguished from unorganized. Living bodies increase in size; so do minerals; the former by the addition of material throughout the tissue, the latter by outside additions. Organized bodies have a limited existence. All are subject to constant change, and to final dissolution. They all spring from a parent, and only originate in this method. The opposite characteristics belong to unorganized matter.
It has been the business of chemistry to determine the ingredients of the earth and atmosphere, by resolving them into what are called elements. By the word element we mean that which can not be resolved into any simpler form. For example, take a piece of chalk; by chemical action it can be divided into a gas called carbonic acid, and a solid called lime; therefore, it is known that chalk is a compound body. If we take a piece of iron or gold there is no process known by which we can resolve it into simpler form, and these are therefore elements. There are over sixty elements and many of them are found in the human body. All these elements are derived from unorganized matter, and the special conditions under which they are formed in organisms is due to the principle of life. The predominant elements that make up the human body are carbon, hydrogen, oxygen, and nitrogen, while iron, potassium, sodium, phosphorous, calcium, exist in smaller quantity. These different elements are variously combined to form compounds, of which water is the most abundant, for it forms more than two-thirds of the entire weight of the body: water is a compound of hydrogen and oxygen. The principal organic compounds are albuminous, of which the white of the egg is a typical example; the gelatinous, or jelly-like compounds, including cartilage, oleaginous, or fatty compounds, and saccharine, or sugary compounds, such as starch. The first two contain nitrogen, hydrogen, oxygen, and carbon, while fat and starch are rich in carbon, but lack nitrogen. In tracing back the development of living matter to its simplest form, it is found to exist as specks without any definite shape or structure, or as granules of extreme minuteness, perhaps the ten thousandth part of an inch in diameter.