Biology, the science which deals with living beings, may be divided into two branches, viz.: i. Morphology, which treats of the form and structure of living creatures; and, 2. Physiology, which attempts to explain the modes of activity exhibited by them during their lifetime, and may therefore be defined as the science which investigates the phenomena presented by the textures and organs of healthy living beings; or, in short, the study of the actions of organisms in contradistinction to that of their shape and structure.

The organic or living world is naturally divided into the Animal and Vegetable kingdoms. We have, therefore, both animal and vegetable morphology and physiology. In studying the vegetable kingdom, the form and structure as well as the activity of plants are associated together to form the subject known as Botany. The physiology of plants need not, therefore, be considered here; though, indeed, a knowledge of it proves useful in considering many of the processes belonging to animal life. On the other hand, the morphology and the physiology of animals are commonly taught separately, and in the medical curriculum are made distinct subjects.

Morphology includes the external form, the general construction or anatomy of organisms, and the minute structure of their textures as revealed by the microscope. The latter branch of study, under the name Histology, has now developed into a very extensive subject, which is inseparable from either physiology or anatomy. In this country histology is commonly taught in the medical schools with physiology, because the time of the teachers of morphology is occupied in expounding the nomenclature of descriptive anatomy, while the microscope is in every-day use in the physiological laboratory. Moreover, an adequate knowledge of microscopic methods, and of the various form elements of the different textures of the body, is one of the first essentials for physiological study.

As the different actions of the body are performed by different tissues, which in the higher animals are grouped together as distinct organs, a general idea of the position and construction of these different parts of the body must be acquired before the study of physiology can be commenced. Anatomy and general morphology are the frameworks upon which physiological knowledge is built up. Some knowledge of these subject's must therefore precede the study of physiology, in order that the student may be in a position to grasp even the simplest facts connected with any physiological question.

We shall soon find that the assistance of other sciences is also indispensable to physiology. Thus every action of a living texture or tissue is accompanied by some chemical change, the chemical process, in fact, being the common essential part of the phenomena of life. The student of physiology must, then, know something of the science of chemistry; indeed, the mode of action of chemical elements forms quite as important a groundwork for the study of the activity of the living tissues as their general form or minute structure.

Further, the laws which govern the motions of inanimate bodies also control the actions of living tissues, for we cannot claim to understand or recognize the existence of any laws affecting living organisms other than those known to be applicable to dead matter. There are a great number of activities shown by living textures which we cannot explain by the recognized laws of chemistry or physics. We therefore use, for convenience' sake, the term "vital phenomena," to indicate processes which are beyond our present chemical and physical knowledge. In using this term we must not think it implies a separate set of natural laws belonging to life. We cannot discover or formulate any special laws affecting living beings only, and therefore we must not assume that any such exist. We must rather endeavor to explain all the so-called "vital phenomena" by means of the laws known to chemists and physicists. By this means we shall certainly get a closer insight into the processes of life, and if there be laws governing the living beings we may learn to know them. This method of working has already given good results, for within comparatively recent times many of the processes which were regarded as specially vital in character have been shown to be within the power of the experimenter and to depend on purely physico-chemical processes.

It is therefore necessary for the physiologist, before he attempts to explain the activities of any organism, to be familiar not only with the structure of its body, but also with the various laws which chemists and physicists teach us control the operations of inanimate matter.

The sciences of chemistry and physics may, in fact, be regarded as the physiology of inorganic matter, just as, when chemistry and physics are applied to the elucidations of the functions of living creatures by the biologist, the study is called physiology. When we consider how far from thoroughly grasping and interpreting all the phenomena presented by the various kinds and conditions of matter the chemist and the physicist still are, we cannot be surprised that those who attempt to explain the actions of living beings find many processes that they are unable to comprehend. So that when physiologists make use of the convenient term "vital phenomena," it must be remembered that they do not thereby imply the existence of a special living force or any kind of energy peculiar to living creatures.

The ultimate object of physiology is not yet within the reach of our modern methods of research. To explain the mode of activity of living beings, and grasp the exact relation borne by their living phenomena to the laws which govern them, is a task of enormous difficulty. Indeed, the manifestations of certain ener gies in living organisms are so complicated that it is often, if not generally, impossible to say exactly how they are brought about, and we are therefore obliged, for the present at least, to be satisfied with the mere recognition and description of the phenomena.

Since the human organism is the special study of students of medicine, the contents of this volume should properly be restricted to the physiology of man. But human physiology cannot be studied alone; because in man we cannot watch sufficiently closely, or question fully, by experiment, the phenomena of life. Further, no sharp line of separation can be drawn between the actions of the various organs of man and those of the lower animals. The consideration of the physiology of those animals which are akin to man must therefore go hand in hand with the study of the physiology of man himself. Much light has been thrown on the actions of the complex textures of the highest animals, by the observation of the activities of the lowest organisms, where the manifestations of life may be carefully watched with the microscope in the living animal under its normal conditions.