rapidity of the circulation. Among the solid constituents, sodic chloride is the most abundant. Lactates, butyrates, and acetates of ammonium and sodium are also present, besides small quantities of calcic phosphate; phosphates of the alkali metals are absent, and the sulphates are found only in very small amount. Carbonic anhydride and nitrogen are likewise exhaled from the surface of the body in considerable quantity, particularly the former. The sweat contains, moreover, a quantity of a peculiar azotised matter, very prone to decomposition, as well as an odorous principle. The cutaneous excretion from the general surface of the body has almost invariably an acid reaction, owing to the presence of free lactic acid; but the excretion from the axillæ and the feet is sometimes found to be alkaline. With regard to the quantity, all that can be stated is that it usually exceeds in bulk that of the urine during the summer; but that in winter the urine preponderates in amount, while the quantity of cutaneous transpiration is greatly and proportionately diminished. In fact, processes occur upon the surface of the skin, which resemble those both of the kidney and of the lungs thus, like the kidneys, the skin excretes nitrogen and acid substances as well as the chlorides; and like the lungs, it separates large quantities of carbonic anhydride and aqueous vapour. The actions of the skin, and of the kidneys and lungs, are, therefore, though to a very limited extent, vicarious; and when the functions of the skin are seriously interfered with, it usually happens that derangements more or less serious, either of the kidneys or of the lungs, occur.

(1710) 3. SOLID EXCREMENTS.-After the chyle has been absorbed into the system during the gradual passage of the mixed constituents of the food through the intestinal canal, those parts which are destined to become excrementitious gradually acquire a fæcal odour, and are at length ejected from the body. The composition of these excrementitious matters necessarily exhibits great variations, which depend mainly upon the nature of the food taken, Berzelius found a specimen of human excrement which he examined, after the use of an ordinary mixed diet, to contain three-fourths of its weight of water, the rest consisting of biliary matter and alimentary débris. A large quantity of phosphates of calcium and of magnesium, and other insoluble salts of the food, is obtained on incinerating the dried residue. Playfair found in a sample which he examined, about 15 per cent. of nitrogen in the dry residue, and 45 per cent. of carbon; but these proportions will necessarily be subject to variation with the kind of food taken.

SOLID EXCREMENTS-PUS.

903

The ratio which these numbers bear to each other does not differ much from that which represents the proportion of the same elements in fresh muscle, which likewise contains three-fourths its weight of water. According to Liebig, the total average weight of feculent matter in its recent state excreted by a healthy adult in twenty-four hours amounts to 5 ounces. The offensive odour of fæcal matter is due to some intermediate products of oxidation. These odorous substances have not been accurately examined, but they may be produced artificially by fusing casein with caustic potash, as in the preparation of tyrosine (1618). This odour is of a different nature and of a more putrid character when the quantity of bile secreted is below the usual amount.

78

2

Dr. W. Marcet has obtained from healthy human excrement a peculiar crystallizable principle which, from its origin, he terms excretine; he attributes to it a composition expressed by the formula ЄH156SO: it has a feebly alkaline reaction; it fuses at a little above 200° (94° C.), and is insoluble in water and in a solution of potash, but is readily soluble in ether. He has also extracted from the excrements a fusible olive-coloured fatty body, which he has named excretolic acid; but he has not recorded any attempt to determine its composition.

(1711) 4. Pus.-This is a liquid which is never met with excepting as a product resulting from actions excited by injury, or by disease. When the suppurative process is going on favourably, pus forms a thick, opaque, yellowish fluid, which, when examined by the microscope, is found to consist of a transparent liquid, in which a large number of corpuscles are seen to be floating. These corpuscles are larger than the red corpuscles in human blood; they consist of a granular cell membrane enclosing a viscid granular matter, and a nucleus which adheres to the membrane. Pus generally has a feebly alkaline reaction. The fluid portion holds a considerable quantity of albumin in solution, and closely resembles ordinary serum leucine is also commonly present (Bödeker). The pus globules contain a large proportion of fat, and of cholesterin. Pyin is the name which has been given to an albuminoid substance sometimes found in pus; it is soluble in water, but is precipitated by acetic acid and by a solution of alum. The dried residue of pus yields a large amount of salts; in the experiments of Lehmann the saline constituents exceeded 12 per cent. of the total dry matter: the proportion of phosphates and of salts of potassium was very considerable.

CHAPTER XIII.

ON THE NUTRITION OF PLANTS AND ANIMALS.

§ I. On the Nutrition of Vegetables.

(1712) ORGANIZED beings have been arranged under the two great divisions of plants and animals; and although the lowest genera in each division approximate so closely that it is difficult to decide where one division begins and the other ends, yet in their general relations they are not only widely different, but even opposed to each other in the functions which they discharge in the economy of creation, the operations of the plant being complementary to those of the animal in the nicely-adjusted balance of organic life, each affording support and nutriment to the other. The principal functions of the plant and animal may be contrasted thus:

Two important functions have been allotted to the vegetable creation, one of which consists in the elaboration from inorganic matter of the various materials necessary to the support of animal life; the other consists in the removal of carbonic anhydride from the atmosphere, the plant retaining the carbon and fixing it in its tissues, whilst the oxygen is restored to the air: during these changes, which require the action of the solar rays, the plant stores up for the future use of man, or of the animal creation, a portion of energy derived from the sun's rays, to be again given out when necessary in the form of heat during combustion, or of muscular force when the vegetable serves as food to the animal. These wonderful changes are produced by the operations of the vegetable cell; each cell possessing a separate and independent power, by means of which it is enabled to assimilate

NUTRITION OF VEGETABLES,

905

new matter; from this assimilated matter it produces new cells, and these in like manner multiply and produce innumerable other cells. These successive developments of vegetable cells may be readily watched under the microscope in the case of the growth of the globules of yeast (1114). The compounds produced in the yeast globules are cellulin, which forms the cell membrane, and the internal substance, which contains a body allied to albumin. The plant-cell in the case of yeast does not elaborate the azotised product, but assimilates it from the solution in which it grows; there is, however, no doubt that albumin is elaborated in other instances by the vegetable cell :-in the wheat plant, for example, the azotised gluten is stored up in the seed, and in this case it can only be derived from the inorganic materials of the soil and of the atmosphere.

But the mere existence of an organized structure, even when animated by vitality, is not sufficient to produce these results of vital activity. It is necessary, in addition, that the plant be exposed to a temperature ranging between 40° and 90°, and to a certain degree of moisture. In the higher manifestations of plant life, solar light is also essential. If at any time the temperature fall so low that the plant is frozen, its vitality is destroyed, and the delicate structures of the tissues are impaired by the solidification and forcible expansion of the juices which they contain. Life is also destroyed not less completely by exposure of the organism to a temperature of 212°, or even to one considerably below that point. The higher orders of plants require for their full development that they shall be rooted in a congenial soil, and be supplied freely with air and moisture.

A plant in a state of health is continually undergoing increase: this increase, however, may either be checked or facilitated by modifying the circumstances under which it is placed. In order that the method of regulating this development may be more fully understood, it is necessary to examine what the materials are of which this increase consists, and whence they are supplied. In all plants there are two distinct sets of substances present; one of these is organic in its nature, and is represented by sugar, starch, wood, albumin, and so on; the other is inorganic, and is represented by the various salts found in the ashes of plants. The materials from which these compounds are formed must, therefore, be supplied in some shape or other to the growing plant; in fact, they constitute its food. Experiment has, however, proved that the form in which these matters are presented to the plant is by no means unimportant, for the elementary

906

NUTRITION OF VEGETABLES.

bodies, with the exception of oxygen, are never directly assimilated; they must previously have entered into combination in some form or other. The compound constituents of the food of plants are supplied from two sources, the atmosphere and the soil. Those furnished by the atmosphere consist chiefly of carbonic anhydride, water, ammonia, and nitric acid; while the saline compounds, which vary with the nature of the plant, are derived from the soil without these different articles of food-viz., carbonic anhydride, water, ammonia, and the salts of the soil— plants could no more continue to exist than animals could do if deprived of their natural nutriment.

The root of a plant may be considered as its mouth, and the leaves as its lungs. When a shower of rain descends through the air, it carries with it all the soluble matters which that portion of the air contains; these soluble matters consist chiefly of carbonic anhydride, with minute quantities of carbonate and nitrate of ammonium. The carbonic anhydride, as we have already stated, is abundantly furnished by the processes of respiration in animals, by combustion, and by a variety of other sources. The ammonia is furnished mainly by the putrefaction and partial oxidation of animal and vegetable matters, and is present in the air, in a quantity far more minute than carbonic anhydride, but still in appreciable amount (369). Nitric acid is produced whenever a flash of lightning passes through the moist air, and this compound is generated even when silent discharges of electricity take place. Every shower of rain which falls to the earth, thus dissolves and carries with it food for vegetation: charged with these nutritive materials, the water percolates through the soil, from which it takes up small quantities of soluble salts. It thus arrives at the spongioles of the roots, where it is absorbed and carried up by the ascending current of sap, to the leaves, and to the remotest extremities of the branches,* where, under the influence of some azotised matter, and of the solar light, the carbonic acid undergoes a remarkable change: a portion of the water likewise experiences decomposition, in consequence of which, oxygen gas, mingled with a quantity of nitrogen varying from one-half to one-tenth

Although it has been stated that the principal supply of carbonic anhy dride is furnished to the plant as a solution in water which is absorbed from the soil by the roots, it must not be forgotten that the leaves also present powerfully absorbent surfaces, and, like the lungs in animals, not only minister to the respiration of the organism, but are frequently the inlet of a variety of substances which are capable of assimilation, as well as of those that are not so. Carbonic anhydride, in particular, is absorbed by the leaves with great rapidity, as has been shown by the experiments of Pepys and others.