This section is from the book "Animal Physiology: The Structure And Functions Of The Human Body", by John Cleland. Also available from Amazon: Animal Physiology, the Structure and Functions of the Human Body.
The Expression Living Parts Of Living Beings, has been already twice used, and will attract the student's attention to the fact that every part of the texture of the body does not equally exhibit the phenomena of life. In a large majority of the different textures, a considerable or even the greater part of the bulk is composed of mere deposited matter, which, although it undergoes both structural and chemical changes, offers no sufficient evidence of the possession of properties peculiar to living beings; but, imbedded in this, or in other instances forming the principal mass of the texture, there is always to be found a set of elements which exhibit some or all of the four functions—nutrition, reproduction, contractility, and irritability.
These living elements of texture always consist of material belonging to one chemical group of substances; namely, those which are termed sometimes the proteids, but which may probably be more conveniently distinguished as the albuminoids, albumen and fibrin being among the most familiar examples of them. The substances of this group are the most complex combinations of carbon, hydrogen, oxygen, and nitrogen; and, as they are found in nature, contain also phosphorus, sulphur, potash, and soda.
Very frequently the expression protoplasm is used to indicate, without much definition, the varieties of albuminoid substance found in the growing stages of the living elements of texture, and in the lowest forms of life.
The generalization has been long known, and may be safely made, that the phenomena of life are never exhibited without the presence of albuminoid substance.
The simplest form of living element in both animal and vegetable texture, or at least one of the simplest forms, and the most important, is the nucleated corpuscle, which is remarkable not only for the remarkable part which it plays, but for its resemblance to some of the simplest kinds of animals, the genus Amoeba.
Fig. 2. Species of Amoeba. After Pritchard.