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*** All quantities, save when otherwise stated, are calculated
to the proportion of 1000.

*** All measurements, save when otherwise stated, are in
vulgar fractions of an English inch.




Physiology studies the phenomena of organised beings, and is therefore a great department of natural science. It is divided into vegetal and animal; and the latter is subdivided into comparative and human, which last branch of the science is the subject of the present volume, the structure or functions of the lower animals being referred to but incidentally.

There is no branch of knowledge whose limits are more hard to define, nor is there any which requires a more extensive acquaintance with preliminary and collateral sciences, such as Anatomy, Physics, and Chemistry; while, on the other hand, so imperceptible is the distinction between most of the normal and abnormal processes, that the physiologist is often compelled to enter the special domain of Pathology. This circumstance is the less to be regretted, as no study can be more fitted to introduce the student of Medicine to a scientific comprehension of the principles of disease, or to free its treatment from empiricism.

All important advances in Pathology have had their rise in accurate observation of, and rational deductions from, physiological phenomena; and disease can be shown to be, in every instance, but an increase, decrease, or perversion of normal function.

Foremost among the sciences which aid the study of Physiology is that of General Anatomy, which for this reason is aptly termed Physiological Anatomy, and is, for convenience sake, usually removed from the study of Descriptive or Surgical Anatomy; in this work it is treated of before the function of each organ is discussed, a short account of its Descriptive Anatomy being however prefixed, when necessary for the comprehension of its actions. To the improvements of the microscope, and the ardour with which it is employed, may be attributed the rapid advance which this branch of Physiology, often termed Histology, has made of late years.

The Vital Phenomena which the human body manifests admit of ready classification into those which it exercises in common with all other organic beings, and those which are peculiar to animals. Such was the view of Bichat when he spoke of the functions of "organic," or vegetative, and of " animal" life. Under the former head may be ranged the processes of digestion, absorption, circulation, respiration, and secretion, which may be grouped together as being subservient to the great function of nutrition; under the latter may be considered the higher endowments of sensation and voluntary motion.

Both organic and animal functions above enumerated relate to the individual alone; but that of reproduction, common both to the vegetal and animal world, has, as its aim, the maintenance of the species.

These views have been borne in mind in arranging the following pages.

We shall here consider the structural elements of which the human body is built up, reserving a more special account of the histology of each organ or tissue until we are describing the functions to which they minister.

In 1888, Schwann and Schleiden demonstrated that the nucleated cell in both animals and plants was the essential structural element in all the tissues, and the active agent in all the organic processes.

A Cell consists of a homogeneous cell-wall and contents, which usually include a nucleus or nuclei, and within these often nucleoli. As the nucleus is regarded as the formative part, the term cytoblast is often applied to it, the matter in which cells are formed, cytoblastema, and the process, cytogenesis. The typical shape is the sphere, but by mutual pressure they usually assume a polyhedral form. A few of the more remarkable shapes which will be illustrated in the ensuing pages may be here mentioned: spheroid cells compose glandular epithelium; regular hexagons the membrana pigmenti; the blood cells are biconcave discs; nerve cells are caudate; muscle cells are fusiform, and the cells of epidermis and tesselated epithelium gradually flatten into scales as they approach the surface.

The Phenomena of Cell-life are various. 1. Growth, by the appropriation of the special material of each kind of cell. 2. Multiplication, either by fission, as can be

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