Under this group may be placed a variety of substances which must be acknowledged to exist in the living tissues as complex chemical compounds, of whose constitution we are ignorant, since it is altered by the death of the tissue.
There are some exceedingly unstable associations of albuminous bodies with other substances, and they at once break up into their more stable constituents, albumins, fats, salts, etc., when they are deprived of the opportunities of chemical interchange and assimilation which are necessary for their life.
Although we can only theorize as to the real chemical constitution of such substances, we must believe that they really exist in the living tissues as chemical compounds, and as chemical compounds endowed with special properties which impart the specific activity of their textures, whose molecular motions, in fact, are the essence of the life of the tissues.
By far the most widely spread and important of these is the soft, jelly-like substance, Protoplasm. This is the really active part of growing textures of all organisms, whether animal or vegetable, and forms the entire mass of those intermediate forms of life, the protista, now generally regarded as the original fountain head of life on the globe.
This material commonly exists in small independent masses (cells), in which we can watch all the manifestations of life, assimilation, growth, motion, etc., taking place. We must assume that this substance is a definite chemical compound; and, further, since the living phenomena are exhibited only so long as it preserves its chemical integrity, we may conclude that its manifestations of life depend upon the sustentation of a special chemical equilibrium. Not only is this equilibrium destroyed by any attempt to ascertain the chemical composition of protoplasm by analysis, but even for its preservation the protoplasm must be surrounded by those circumstances which are known to be necessary for. life, viz., moisture, warmth, and suitable nutritive material, or its destruction must be warded off by a degree of cold that checks its chemical activity.
If the chemical integrity of protoplasm be destroyed and its death produced, many new substances appear, among which are representatives of each of the great chemical groups found in the animal tissues. Thus, besides water and inorganic salts, we find in protoplasm carbohydrates represented by glycogen, lecithin and other fats, and several albuminous bodies, which will 6 be described in the groups to which they belong. In addition to these, protoplasm often contains some foreign bodies which have come from without, and special ingredients of its own manufacture, such as oil, pigment, starch and chlorophyll.
There is in living blood also a body which must be included in this group, as it undoubtedly has a much more complex constitution than any of the individual albuminous bodies, presently to be described, which can be obtained from it. This is proved by the following facts: first, its death is accompanied by a series of chemical changes, viz., disappearance of free oxygen, diminution of alkalinity, and a rise in temperature, and secondly, certain albuminous bodies appear which were not present in the living plasma.
The spontaneous decomposition of separated blood plasma may be delayed by cold: at freezing point the chemical processes are held in check. During life the exalted constitution of the plasma is sustained by certain chemical interchanges which go on between it and its surroundings. This question will be more fully discussed when the coagulation of the blood is described.
Likewise, as will be found in the chapter on Muscles, there exists in the soft, contractile part of striated muscle a plasmas which at its death spontaneously breaks up into other distinct albuminous bodies and forms a coagulum. These changes are accompanied by acidity of reaction, the disappearance of oxygen and an elevation of temperature, showing that distinct chemical change is taking place.
Oxyhemoglobin, the coloring matter of the blood, should be included here among the important chemical bodies more complex than the albumins. This singular, body can be broken up into a globulin and a coloring matter, haematin, containing iron. It differs from all other bodies of a similarly complex nature from the fact that it readily crystallizes, and also in the very remarkable manner in which it combines with oxygen, and again yields it up.