satisfy the rules of modern chemistry, because their classification, from a strictly chemical point of view, does not set forth their physiological importance or express adequately the relation they bear to the vital phenomena of organisms. The following enumeration of the chief chemical ingredients found in the tissues has regard to their physiological dignity as well as to their chemical construction, and will thus, it is hoped, assist the student to distinguish the different groups, and give him a better idea of their vital relationships, than a more strictly systematic classification. (A) NITROGENOUS. I. Complex bodies forming the active portion of many tissues-Plasmata, e. g., protoplasm, blood plasma. II. Bodies entering into the formation of and easily obtained by analysis from Group I, Albumins, e. g., serum albumin. III. Bodies the outcome of differentiation, manufactured in the tissues by Group I, Albuminoids, e. g., gelatin, etc. IV. Bodies containing nitrogen, being intermediate, bye, or effete products of tissue manufacture, e. g., lecithin, urea, etc. (B) NON-NITROGENOUS. V. Carbohydrates in which the hydrogen and oxygen exist in the proportion found in water, e. g., starch and sugar. VI. Substances containing oxygen in less proportions than the above, e. g., fats. VII. Salts. VIII. Water. CLASS A.-NITROGENOUS. GROUP I.-PLASMATA. 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. Protoplasm.-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 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. Blood Plasma.-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. Muscle Plasma.-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, hæmatin, 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. GROUP II-ALBUMINOUS BODIES. It is difficult to say how far these bodies exist as such in the living organism, but they can be obtained from nearly all parts, particularly those which contain active protoplasm, and after its death they can be detected in abundance. As may be seen, by testing for their presence in living protoplasm, the addition of any chemical reagent or treatment causes its death, so that, although albumins appear in the test tube, this cannot be accepted as proof that they would have answered to the tests before the protoplasm was changed by its death. They do not occur normally in any secretion except those substances which tend to nourish the adult body, and to form and nourish the offspring, viz., the ovum, semen and milk. No satisfactory formula has been suggested to express their chemical composition, but the average percentage of the elements they contain is remarkably alike in all members of the group. This may be said to be in round numbers as follows: They are amorphous, of varying solubility, and, with one exception, indiffusible in distilled water. As far as we know at present, albumins cannot be constructed de novo in the animal body, but must be supplied in one form or another as part of the food. Albumins are therefore always the outcome of the activity of vegetable life. They can be recognized by the following tests : 1. Strong nitric acid gives a pale yellow color to solutions or 3. Solution of caustic soda and a drop of copper sulphate 4. Acetic acid and boiling give a white precipitate, except with derived albumins and peptones. 5. Acetic acid and potassium ferrocyanide give a flocculent white precipitate, except with peptones. 6. Acetic acid and equal volumes of sodium sulphate solution give a precipitate on boiling. 7. With sugar and sulphuric acid they become violet. 8. Crystals of picric acid added to solutions dissolve and cause bead-like local coagulations, except with peptones. CLASSIFICATION OF ALBUMINS. Under the head of the albuminous bodies we find several classes which differ from each other in slight but very important points. The first class may be called (A) ALBUMINS PROPER, OR NATIVE ALBUMINS. They consist of 1. Egg Albumin, which does not occur in the ordinary tissues of the animal, can be procured by filtration from the white of an egg. It makes a clear or slightly opalescent solution in water, from which it is precipitated by mercuric chloride, silver nitrate, lead acetate, and alcohol. It is coagulated by heat, strong nitric and hydrochloric acids, or prolonged exposure to alcohol or ether. 2. Serum Albumin, on the other hand, is one of the chief forms of albumin found in the nutrient fluids. It differs from egg albumin in (a) Not coagulating with ether. (b) The precipitate obtained by strong hydrochloric acid (c) Coagulum being more readily soluble in nitric acid. (e) If introduced into the circulation, it is not eliminated (B) GLOBULINS. Associated with the last during the life of the tissues we find another class of albumins, namely, the globulins, which do not |