pearance of sugar in the urine. Schiff' has even found that in various animals, by simply compressing the abdominal aorta for ten minutes, or by tying the principal bloodvessels of one limb, he has induced, for the time, a condition of diabetes. These different causes may all operate by accelerating the hepatic circulation as well as that of the abdominal organs generally.

III. A saccharine condition of the blood and urine may also be induced by puncture of the medulla oblongata in the floor of the fourth vertricle. This remarkable fact, which was first discovered by Bernard, may be demonstrated in both carnivorous and herbivorous animals. It is best shown in the rabbit by introducing a narrow chisel-shaped instrument, with the cutting edge directed transversely, through the back part of the skull and the cerebellum, so that it shall pierce the posterior part of the medulla exactly in the median line, without passing completely through its substance. Glucose appears in the urine after from one to two hours and continues to be present for two or three days. The immediate effect of this operation, according to the direct observations of Bernard, is to increase the activity of the abdominal and hepatic circulation. It is not due to a direct influence conveyed by the pneumogastric nerve, since the result follows, as usual, although the pneumogastric nerves may have been divided, and neither division nor irritation of these nerves produces a similar effect. When successfully performed, the operation causes no serious disturbance of the vital functions, and the animal recovers after a few days without suffering permanent injury.

In all the instances above mentioned, the appearance of sugar in the urine is only temporary, depending upon an occasional disturbance of the circulation. When in the human subject this condition becomes permanent, it constitutes the disease known as Diabetes mellitus. In this affection, which is generally progressive and fatal, the urine is increased in quantity, of greater specific gravity than natural, and continuously charged with sugar, sometimes in excessive abundance. Fluctuations are observable in the quantity of glucose discharged at different periods of the digestive process, but it may continue to appear, even when no starchy or saccharine matter is taken with the food.

1 Journal de l'Anatomie et de la Physiologie. Paris. 1866. No. iv. p. 365. 2 Leçons de Physiologie Expérimentale. Paris, 1855, p. 290.

CHAPTER XII.

THE BLOOD.

THE blood, in its natural condition, while circulating in the vessels, is a thick opaque fluid, varying in different parts of the body from a brilliant scarlet to a dark purple or nearly black color. It has a slightly alkaline reaction, and a specific gravity of 1055. It consists, first, of a nearly colorless, transparent, alkaline fluid, termd the plasma, containing water, fibrine, albumen, and salts, in a fluid condition; and, secondly, of a large number of distinct cells, or corpuscles, the blood-globules, swimming freely in the liquid plasma. The globules form about 40 per cent., and the plasma about 60 per cent., by volume, of the entire mass. The specific gravity of the two ingredients is somewhat different. That of the plasma is about 1030; that of the globules, 1088. Their relative quantities, by weight, are therefore more nearly equal to each other than when estimated by volume; the exact proportions, according to Robin, being nearly 45 per cent. of globules and 55 per cent. of plasma.

Notwithstanding the difference in specific gravity between the bloodglobules and the plasma, the natural movement of the blood in the vessels keeps them thoroughly mingled; and even when the blood is allowed to remain at rest in a glass jar, the globules subside only very slowly and imperfectly. Thus the globules, disseminated uniformly throughout the plasma, give to the entire mass of the blood an opaque aspect and a deep red color.

The globules of the blood are of two kinds, namely, red and white; of these the red are by far the most numerous.

Red Globules of the Blood.

The red globules of human blood are so abundant that, in the thinnest layer under the microscope, they appear crowded together in such profusion as to cover or touch each other in every direction. According to the estimates of Welcker and Vierordt about 5 millions of them are contained in each cubic millimetre of blood. On account of their quantity therefore, as well as their peculiar properties, it is evident that they form a most important constituent of the circulating fluid.

Physical Properties of the Red Globules.-The red globules of human blood present, under the microscope, a perfectly circular outline and a smooth exterior. According to the most recent and careful measurements of various observers, they have, on the average, a transverse diameter of from 7.50 to 7.75 mmm. Their size varies more or less, but this variation is not very marked for the greater number of the ( 243 )

globules, and, according to the observations of Schmidt, over 90 per cent. of those contained in a single specimen have the same dimensions. The smallest size observed is 4.50 mmm. (Harting), and the largest 9.3

b..

.

Fig. 76.

HUMAN BLOOD-GLOBULES.-a Red glob ules, seen flatwise. b. Red globules, seen edgewise. c. White globule.

mmm.; while their average diameter, as found in different individuals, varies from 6.70 to 8.20 mmm.

The form of the red globule is that of a spheroid, very much flattened on its opposite surfaces, somewhat like a thick piece of money with rounded edges. The globule accordingly, if seen flatwise, presents a comparatively broad surface and a circular outline (Fig. 76, a); but if it be made to roll over, it will present itself edgewise during its rotation, and assume the flattened form indicated at b. The thickness of the globule, seen in this position,

is about one-fifth of its transverse diameter. When the globules are examined lying upon their broad surfaces, it can be seen that these

Fig. 77.

surfaces are not exactly flat, but that there is on each side a slight central depression, so that the rounded edges of the bloodglobule are evidently thicker than its middle portion. This inequality produces a remarkable optical effect. The substance of which the blood-globule is composed refracts light more strongly than the fluid plasma. Therefore, when examined with the microscope by transmitted light, the thick edges of the globules act as double convex lenses, and concentrate the light above the level of the fluid. Consequently, if the object-glass be carried upward by the adjusting screw of the microscope, and lifted away from the stage, so that the blood-globules fall beyond its focus, their edges will appear brighter. But the central portion of each globule, being excavated on both sides, acts as a double concave lens, and disperses the light from a point be

KED GLOBULES OF THE BLOOD, seen a little beyond the focus of the microscope.

Fig. 78.

low the level of the fluid. It thus becomes brighter as the objectglass is carried downward, and the object falls within its focus. An alternating appearance of the blood-globules may, therefore, be produced by viewing them first beyond and then within the focus of the instrument. When beyond the focus, the globules will be seen with a bright rim and a dark centre (Fig. 77). When within it, they will appear with a dark rim and a bright centre. (Fig. 78.)

Within a minute after being placed under the microscope, the blood-globules, after a fluctuating movement of short duration, often arrange themselves in slightly curved rows or chains, in which they adhere to each

THE SAME, seen a little within the focus.

Fig. 79.

other by their flat surfaces, presenting an appearance which has been aptly compared with that of rolls of coin. This is probably owing to the coagulation of the blood, which takes place very rapidly when spread out in thin layers and in contact with glass surfaces; and which, by compressing the globules, forces them into such a position that they occupy the least possible space.

The color of the blood-globules, when viewed by transmitted light and in a thin layer, is a light amber or pale yellow. It is deep red when seen by reflected light, or in thick layers. The blood-globules have a consistency which is very nearly fluid They are exceedingly flexible, and easily elongated, bent, or distorted by pressure in passing through the narrow currents of fluid which often establish themselves in a drop of blood under microscopic examination; but regain their original shape as soon as the pressure is taken off.

RED GLOBULES OF THE BLOOD, adhering together, like rolls of coin.

So far as immediate observation can show, the red globules of the blood, in man and the mammalians, are homogeneous in structure. The most careful examination fails to show, with any certainty, the evidence

of an external envelope, distinct from the parts contained within it; and although some microscopists of high authority (Kölliker, Richardson) continue to regard the existence of such a cell-membrane as probable, it is not generally admitted, and cannot be directly demonstrated.

Fig. 80.

Each globule appears to consist of a mass of organic substance, presenting the same color, consistency, and composition throughout.

The appearance of the bloodglobules is altered by various physical and chemical reagents. If a drop of blood, when placed under the microscope, be not protected from evaporation, the globules near the edges of the preparation often diminish in size, becoming shrivelled and crenated, presenting an appearance as if minute granules were projecting from their surfaces; an effect apparently produced by the loss of a part of their watery ingredients. This distortion of the globules sometimes takes place with great rapidity, and care is requisite not to confound a change produced by external physical causes with morbid alteration of the ingredients of the blood. According to the observations of Kölliker, this, as well as certain other abnormal forms presented by the blood-globules, is never to be seen in the blood while circulating in the vessels.

RED GLOBULES OF THE BLOOD, shrunken, with their margins crenated.

If water, on the other hand, be added to the blood, so as to dilute the plasma, the red globules absorb it by imbibition, lose the central concavity of their flat surfaces, assume the spherical form, and become paler. If a larger quantity of water be added, it may dissolve out completely the coloring matter, leaving the globules as pale, colorless circles, which are almost invisible on account of their tenuity. They may still, however, be brought into view by the addition of an iodine solution, which stains them of a yellowish color. If the water added to the blood be moderate in quantity, just sufficient to be taken up by imbibition by the globules, but not to extract their coloring matter, a special change in their form is exhibited. The edges of the globules, being thicker than the central portions, and absorbing water more abundantly, become turgid, and encroach gradually upon the central part. (Fig. 81.) It is very common to see the central depression, under these circumstances, disappear on one side before it is lost on the other, so that the globule, as it swells up, curls over toward one side, and assumes a peculiar cup-shaped form. (Fig. 81, a, a.) This figure may often be seen in blood-globules after soaking for some time in the urine,