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histology that is frequently designated by the specific name of cytology), general histology, and gross anatomical dissection are therefore frequently employed in physiological investigations, and form what may be called the observational side of the science. On the other hand, we have the experimental methods, that seek to discover the properties and functional relationships of the tissues and organs by the use of direct experiments. These experiments may be of a surgical character, involving the extirpation or destruction or alteration of known parts by operations upon the living animal, or they may consist in the application of the accepted methods of physics and chemistry to the living organism. The physical methods include the study of the physical properties of living matter and the interpretation of its activity in terms of known physical laws, and also the use of various kinds of physical apparatus such as manometers, galvanometers, etc. for recording with accuracy the phenomena exhibited by living tissues. The chemical methods imply the application of the synthetic and analytic operations of chemistry to the study of the composition and structure of living matter and the products of its activity. The study of the subjective phenomena of conscious life-in fact, the whole question of the psychic aspects or properties of living matter-for reasons that have been mentioned is not usually included in the science of physiology, although strictly speaking it forms an integral part of the subject. This province of physiology has, however, been organized into a separate science, psychology, although the boundary line between psychology as it exists at present and the scientific physiology of the nervous system cannot always be sharply drawn.

It follows clearly enough from what has been said of the methods used in animal physiology that even an elementary acquaintance with the subject as a science requires some knowledge of general histology and anatomy, human as well as comparative, of physics, and of chemistry. When this preliminary training is lacking, physiology cannot be taught as a science; it becomes simply a heterogeneous mass of facts, and fails to accomplish its function as a preparation for the scientific study of medicine. The mere facts of physiology are valuable, indeed indispensable, as a basis for the study of the succeeding branches of the medical curriculum, but in addition the subject, properly taught, should impart a scientific discipline and an acquaintance with the possible methods of experimental medicine; for among the so-called experimental branches of medicine physiology is the most developed and the most exact, and serves as a type, so far as methods are concerned, to which the others must conform.

II. BLOOD AND LYMPH.

BLOOD.

A. GENERAL PROPERTIES: PHYSIOLOGY OF THE CORPUSCLES.

THE blood of the body is contained in a practically closed system of tubes, the blood-vessels, within which it is kept circulating by the force of the heartbeat. The blood is usually spoken of as the nutritive liquid of the body, but its functions may be stated more explicitly, although still in quite general terms, by saying that it carries to the tissues food-stuffs after they have been properly prepared by the digestive organs; that it transports to the tissues oxygen absorbed from the air in the lungs; that it carries off from the tissues various waste products formed in the processes of disassimilation; that it is the medium for the transmission of the internal secretion of certain glands; and that it aids in equalizing the temperature and water contents of the body. It is quite obvious, from these statements, that a complete consideration of the physiological relations of the blood would involve substantially a treatment of the whole subject of physiology. It is proposed, therefore, in this section to treat the blood in a restricted way-to consider it, in fact, as a tissue in itself, and to study its composition and properties without special reference to its nutritive relationship to other parts of the body.

Histological Structure. The blood is composed of a liquid part, the plasma, in which float a vast number of microscopic bodies, the blood-corpuscles. There are at least three different kinds of corpuscles, known respectively as the red corpuscles; the white corpuscles or leucocytes, of which in turn there are a number of different kinds; and the blood-plates. As the details of structure, size, and number of these corpuscles belong properly to textbooks on histology, they will be mentioned only incidentally in this section when treating of the physiological properties of the corpuscles. Blood-plasma, when obtained free from corpuscles, is perfectly colorless in thin layers-for example, in microscopic preparations; when seen in large quantities it shows a slightly yellowish tint, the depth of color varying with different animals. This color is due to the presence in small quantities of a special pigment, the nature of which is not definitely known. The red color of blood is not due, therefore, to coloration of the blood-plasma, but is caused by the mass of red corpuscles held in suspension in this liquid. The proportion by bulk of plasma to corpuscles is usually given, roughly, as two to one.

Blood-serum and Defibrinated Blood.-In connection with the explanation of the term "blood-plasma " just given, it will be convenient to define briefly the terms "blood-serum" and "defibrinated blood." Blood, after it escapes from the vessels, usually clots or coagulates; the nature of this process is

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discussed in detail on p. 54. The clot, as it forms, gradually shrinks and squeezes out a clear liquid to which the name blood-serum is given. Serum resembles the plasma of normal blood in general appearance, but differs from it in composition, as will be explained later. At present we may say, by way of a preliminary definition, that blood-serum is the liquid part of blood after coagulation has taken place, as blood-plasma is the liquid part of blood before coagulation has taken place. If shed blood is whipped vigorously with a rod or some similar object while it is clotting, the essential part of the clot― namely, the fibrin-forms differently from what it does when the blood is allowed to coagulate quietly; it is deposited in shreds on the whipper. Blood that has been treated in this way is known as defibrinated blood. It consists of blood-serum plus the red and white corpuscles, and as far as appearances go it resembles exactly normal blood; it has lost, however, the power of clotting. A more complete definition of these terms will be given after the subject of coagulation has been treated.

Reaction. The reaction of blood is alkaline, owing mainly to the alkaline salts, especially the carbonates of soda, dissolved in the plasma. The degree of alkalinity varies with different animals: reckoned as Na,CO,, the alkalinity of dog's blood corresponds to 0.2 per cent. of this salt; of human blood, 0.35 per cent. The alkaline reaction of blood is very easily demonstrated upon clear plasma free from corpuscles, but with normal blood the red color prevents the direct application of the litmus test. A number of simple devices have been suggested to overcome this difficulty. For example, the method employed by Zuntz is to soak a strip of litmus-paper in a concentrated solution of NaCl, to place on this paper a drop of blood, and, after a few seconds, to remove the drop with a stream of water or with a piece of filterpaper. The alkaline reaction becomes rapidly less marked after the blood has been shed; it varies also slightly under different conditions of normal life and in certain pathological conditions. After meals, for instance, during the act of digestion, it is said to be increased, while, on the contrary, exercise causes a diminution. In no case, however, does the reaction become acid. For details of the methods used for quantitative determinations of the alkalinity of human blood, reference must be made to original sources.1

Specific Gravity.-The specific gravity of human blood in the adult male may vary from 1041 to 1067, the average being about 1055. Jones' made a careful study of the variations in specific gravity of human blood under different conditions of health and disease, making use of a simple method which requires only a few drops of blood for each determination. He found that the specific gravity varies with age and sex, that it is diminished after eating and is increased by exercise, that it falls slowly during the day and rises gradually during the night, and that it varies greatly in individuals, "so much so that a specific gravity which is normal for one may be a sign of disease in another." The specific gravity of the corpuscles is slightly greater

1 Wright: The Lancet, 1897, p. 8; Winternitz: Zeitschrift für physiol. Chemie, 1891, Bd. 15,

S. 505.

Journal of Physiology, 1891, vol. xii., p. 299.

than that of the plasma. For this reason the corpuscles in shed blood, when its coagulation is prevented or retarded, tend to settle to the bottom of the containing utensil, leaving a more or less clear layer of supernatant plasma. Among themselves, also, the corpuscles differ slightly in specific gravity, the red corpuscles being heaviest and the blood-plates being lightest.

Red Corpuscles.-The red corpuscles in man and in all the mammalia, with the exception of the camel and other members of the group Camelidæ, are biconcave circular disks without nuclei; in the Camelidæ they have an elliptical form. Their average diameter in man is given as 7.7μ (1μ = 0.001 of a mm.); their number, which is usually reckoned as so many in a cubic millimeter, varies greatly under different conditions of health and disease. The average number is given as 5,000,000 per cubic mm. for males and 4,500,000 for females. The red color of the corpuscles is due to the presence in them of a pigment known as "hæmoglobin." Owing to the minute size of the corpuscles, their color when seen singly under the microscope is a faint yellowish-red, but when seen in mass they exhibit the well-known blood-red color, which varies from scarlet in arterial blood to purplish-red in venous blood, this variation in color being dependent upon the amount of oxygen contained in the blood in combination with the hæmoglobin. Speaking generally, the function of the red corpuscles is to carry oxygen from the lungs to the tissues. This function is entirely dependent upon the presence of hæmoglobin, which has the power of combining easily with oxygen gas. The physiology of the red corpuscles, therefore, is largely contained in a description of the properties of hæmoglobin.

Condition of the Hæmoglobin in the Corpuscle.-The finer structure of the red corpuscle is not completely known. It is commonly believed that the corpuscle consists of two substances-a delicate, extensible, colorless protoplasmic material, which gives to the corpuscle its shape and which is known as the stroma, and the hæmoglobin. The latter constitutes the bulk of the corpuscle, forming as much as 95 per cent. of the solid matter. It was formerly thought that hæmoglobin is disseminated as such in the interstices of the porous spongy stroma, but there seem to be reasons now for believing that it is present in the corpuscles in some combination the nature of which is not fully known. This belief is based upon the fact that Hoppe-Seyler1 has shown that hæmoglobin while in the corpuscles exhibits certain minor differences in properties as compared with hæmoglobin outside the corpuscles. In various ways the compound of hæmoglobin in the corpuscles may be destroyed, the hæmoglobin being set free and passing into solution in the plasma. Blood in which this change has occurred is altered in color and is known as "laky blood." In thin layers it is transparent, whereas normal blood with the hæmoglobin still in the corpuscles is quite opaque even in very thin strata. Blood may be made laky by the addition of ether, of chloroform, of bile or the bile acids, of the serum of other animals, by an excess of water, by alternately freezing and thawing, and by a number of other methods. In connection with two of these methods of discharging hæmoglobin from the Zeitschrift für physiologische Chemie, Bd. xiii., 1889, S. 477.

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