spectrum with that of phyllocyanin (see fig. 51, iii.); dissolve the rest in hot alkali; add excess of acetic acid; a precipitate is produced; shake up with ether to dissolve this, 'allow it to stand for several days; then compare the spectrum with that in fig. 51, v. If these spectra are seen and identified, the colouring matter under examination is certainly chlorophyll. Functions of chlorophyll.-Schunck concludes his memoir on chlorophyll by many interesting suggestions as to its chemical constitution. I here merely mention one of these. He considers that carbonic acid is one of the constituents of the molecule, but that it is held more loosely than in an ordinary carbonate, and yet in a state of greater condensation than it would be in a mere watery solution. It is thus in a favourable condition for transfer to the assimilating plasma, which effects its decomposition with elimination of oxygen, and the chlorophyll would then be in a state to take up fresh quantities of carbonic acid, acting therefore as a carrier of carbonic acid in the plant, just as hæmoglobin serves to convey oxygen in the animal economy. Chlorophyll is always present in vegetable cells in which the formation of organic matter from carbonic acid and water, with elimination of oxygen, is going on. Parasitic plants like fungi obtain their nutriment ready formed from other organisms or decaying organic matter. They contain no chlorophyll, and do not decompose CO, as other plants do in the light. Plants grown in darkness from seeds or tubers contain no chlorophyll, and die when the food material stored in the seed or tuber is exhausted. The appearance of chlorophyll in etiolated plants on exposure to light indicates the commencement of assimilation. It is therefore certain that chlorophyll plays some part in the process of assimilation, but how it acts in assisting the process is unknown. In the green cells of plants, the chlorophyll is found associated with proteid masses which it permeates and tinges green, forming the socalled chlorophyll corpuscles or granules, and probably the power of decomposing carbonic acid and water with evolution of oxygen resides in the chlorophyll corpuscle rather than in the pigment simply. Jumelle considers that chlorophyll assists in the transpiration of water from the leaves of plants. 1 Compt. rend. Soc. Biol. 1889, p. 9. CHAPTER XV THE BLOOD THE blood forms a very convenient starting-point for a consideration of the chemistry of the elementary tissues. Using the word tissue in the sense of texture, some would, perhaps, hesitate to include the blood under that head; but using the word in the sense of elementary principle, there seems but little reason why we should not include the blood with epithelium, muscle, &c., among the tissues. Many of the other tissues, such as muscle, are composed of a semi-fluid material, and there are but few parts of the body that do not contain a large percentage of water; blood is certainly the most fluid of the tissues, but floating in its fluid matrix are a large number of more solid particles, or blood corpuscles, which are analogous to the cellular elements of the other tissues. From an embryonic point of view, the blood is most nearly allied to the group of connective tissues; it is developed in connection with certain mesoblastic cells in situations where connective tissues are in process of formation, and its cellular elements are throughout life reinforced by the multiplication of cells which are situated also in various connective tissue structures (lymphoid tissue, marrow of bone). The blood is not only distinguished from other tissues by its greater fluidity, but also by the fact that throughout life it is in continual movement. This movement constitutes what is called the circulation of the blood. Speaking generally, the functions of the blood consist in ministering to the needs of the other tissues. It receives oxygen from the air and conveys it to the tissues and organs generally; it receives nutrient material from the alimentary canal, and this also it carries to the rest of the body. In return, it receives from the other tissues the products of their combustion, and conveys them to organs such as the lungs and kidneys, where they are finally got rid of or excreted. The blood thus comes into relation with all the organs, and plays an important part in respiration, nutrition, and in all the other functions of the body. In those animals in which there is but little or no differentiation of function, there is no circulating fluid to bring the different parts into relation with one another, and at the other extreme of the animal kingdom, where we find the greatest complexity, it is there also that we find the blood in its most highly developed condition. We shall find it most convenient to take the consideration of the blood in vertebrates first, and leave that of the different invertebrate classes for a subsequent chapter. This order will be the best, first, because our knowledge of vertebrate blood is more complete, and, secondly, from the point of view of human physiology and pathology it is of more practical importance. Colour.-In vertebrates the blood is a somewhat viscous and, to the naked eye, homogeneous red liquid. The tint varies according to the state of oxygenation of the pigment hæmoglobin, to which the colour of the blood is due. The blood which leaves the lungs or, in aquatic animals, the gills, is of a bright scarlet hue, while that in the systemic veins is purplish. In contact with the air a loose combination called oxyhemoglobin is formed which is scarlet, and in the tissues this oxygen is in great measure given up, and the blood returning to the heart has the darker purplish tint of hæmoglobin. In only two vertebrate animals,' Amphioxus, or the lancelet, and Leptocephalus, another small fish, the blood contains no hæmoglobin, and is colourless. Microscopic investigation of vertebrate blood shows that it is not a homogeneous red liquid, but that it consists of a nearly colourless liquid, the plasma or liquor sanguinis, holding in suspension large numbers of solid bodies the corpuscles. These corpuscles are of two kinds the coloured and the colourless. It is in the former, the coloured or red corpuscles, that the pigment hæmoglobin is contained. The plasma, however, does in many cases contain a pigment in solution, or perhaps more than one. These will be treated of in connection with the serum. Specific gravity. Roy2 has introduced a method for ascertaining the specific gravity of living blood. A drop of blood from the finger is received into a mixture of glycerine and water of known specific gravity. If the drop tends to rise or sink it is assumed that it is of lower or higher specific gravity than the fluid in which it is placed. By having ready to hand a number of such standard solutions of glycerine and water of different specific gravities, it is not difficult to find one in which the blood neither rises nor sinks, and, as its specific gravity is known, the specific gravity of the blood under examination is also ascertained. The average specific gravity of human blood thus found is 1060. Lloyd Jones finds that with a little practice this proceeding can be carried out very quickly, and from the examination of a large number of cases concludes 1 Lankester, Proc. Roy. Soc. vol. xxi. 1872, p. 71, et seq. 2 Roy, Proc. Physiol. Soc. 1884. 5 E. L. Jones, Journ. of Physiol. vol. viii. (1887), p. 1.' that there is a ‘diurnal variation' in the specific gravity of the blood, consisting of a fall during the day and a rise during the night. The specific gravity of the blood is higher in the male than in the female; and that during pregnancy, after exercise, or after the ingestion of food, there is a fall. In a passively congested part the specific gravity of the blood is high. The specific gravity of defibrinated blood varies considerably, the average for human blood being 1055 (Becquerel and Rodier)'; for dog's blood, 1060 (Pflüger); for rabbit's blood, 1042 to 1052 (Gschleidlen).3 The specific gravity falls in anæmia and wasting diseases generally. It also falls after hæmorrhage. The specific gravity of defibrinated blood may be ascertained by the use of the hydrometer, or more correctly by actual weighing (see p. 15). Reaction. The reaction of vertebrate blood is always alkaline. This is due to the alkaline salts which are present. The demonstration of the alkalinity of the blood is very simple. A drop of blood is placed on the smooth, faintly reddened surface of a piece of dry, glazed litmus paper, and after a few seconds is wiped off with a piece of clean linen rag moistened with water. The place where the blood has been standing is marked out as a well-defined blue patch (Schäfer 3). The manufacture of glazed litmus paper of the kind just alluded to renders unnecessary the somewhat elaborate methods adopted by older observers to demonstrate the alkalinity of the blood. Thus Kühne placed the blood in a small dialyser suspended in a watch-glass full of water; some of the salts pass into the water, the alkalinity of which can be then shown. Liebreich' recommended porous slabs of plaster of Paris coloured by neutral litmus instead of litmus paper, and Zuntz3 used litmus paper previously moistened with a strong solution of sodium sulphate or sodium chloride. The alkalinity of plasma or of serum, where there is no difficulty arising from the presence of a mass of deeply coloured corpuscles, can be always demonstrated by the use of ordinary litmus paper, or of litmus solution. Taste and odour.-The salts present in the blood give it a saline taste. Blood has also a slight but peculiar odour dependent on the presence of minute quantities of volatile fatty acids. This odour, known as the halitus sanguinis, differs in different animals. It may be further 1 Becquerel and Rodier, Recherches sur les altérations du sang, Paris, 1844. Traité de chimie pathologique, Paris, 1854, p. 41. Pflüger, Pflüger's Archiv, i. 75. 3 Gschleidlen, quoted in Gamgee's Physiological Chemistry, p. 26. * Such as are prepared by Messrs. Townson & Mercer, Bishopsgate Street. * E. A. Schäfer, Journal of Physiology, vol. iii. 6 Kühne, Virchow's Archiv, vol. xxxiii. (1865), p. 95. 7 Liebreich, Berichte d. deutschen chem. Ges. zu Berlin, 1868, p. 48. * Zuntz, Centralbl. f. d. med. Wissensch. 1867, no. 34. developed by adding to the blood a mixture of equal parts of sulphuric acid and water. 14 Quantity of blood in the body.-This averages to of the total body weight. The most accurate method of estimating the total amount of blood in the body is Welcker's. It may be briefly described as follows: A small quantity of blood is removed from the animal by venesection, defibrinated, measured, and diluted to known extents to serve as standards of comparison. The animal is then bled to death; the blood is defibrinated. The vessels are next washed out with water or saline solution, the washings added to the blood; lastly, the whole animal is finely minced with water or saline solution, the extract is filtered and added to the diluted blood previously obtained, and the whole is measured. The colour of the mixture is then compared with the standard solutions made from the few cubic centimetres of blood which were first removed, until one is discovered which has the same tint as the mixture. The amount of blood in the corresponding standard solution being known, the total quantity in the animal's body can in that way be easily calculated. COAGULATION OF THE BLOOD Within a few minutes after the blood has been shed it becomes viscous, and then rapidly sets into a solid red jelly. The formation of the jelly begins on the surface of the liquid and on the sides of the vessel in which it is contained; this rapidly spreads through the whole substance of the liquid. In a few minutes after this, drops of a more the upper surface of or less faintly straw-coloured liquid appear upon the jelly; these become larger and run together; the jelly shrinks from the sides of the containing vessel, more fluid collects, and ultimately the clot floats in a liquid. The appearance of this liquid, which is called serum, is due to the shrinking of one of the constituents of the clot, called fibrin, and the process of shrinking may go on for twelve to We can thus distinguish two steps in the coagulatwenty-four hours. tion of the blood: : 1. The stage of jellying. 2. The shrinkage of the clot, and the consequent expression of the serum. of With the microscope more details can be made out than with the naked eye. Filaments of fibrin are seen forming a network; many these radiate from small clumps of blood tablets. These blood tablets See also Gschleidlen, |