The blanching of the leaves has been chosen as an illustrative example of the various causes which may underlie what is apparently but one dis ease, because this change in colour bears a close resemblance to that which gives rise to the popular conception of anemia. And not only this, but in both of these manifestations there is, in so far as a comparison can be drawn between animals and plants, a similar pathological process underlying each. The green colour of the leaves is due to a colouring agent, the chlorophyll, just as the red colour of the blood is due to a similar agent, the haemoglobin.
The blood consists not only of an albuminous fluid, in which are con tained the numerous salts and other substances necessary to the growth of the body, but it also contains an enormous number of minute cells of various forms and sizes, which, according to their colour, are described as red or white blood-corpuscles. The red cells, which are far more numerous than the white, contain the red colouring-matter, which gives to the blood and the tissues supplied by it their characteristic colour. This substance permits the red cells to absorb and retain the oxygen supplied to the body through the agency of the respiratory organs. The more completely this is done, the more intense is the red colour of the corpuscles, and the greater their ability to carry out the metabolic exchange to the welfare of the tissues in the body. A rugged, blooming colour of the skin may therefore with entire justice be looked upon as an evidence of health. The complexion does not, however, depend entirely on the number, nor on the intensity of the colour, of these corpuscles, but also on the degree of dilatation of the blood-vessels in the skin and on the transparent and tender character of its layers. For these reasons it is apparent that changes in the complexion are dependent upon : (r) a decrease in the number of red blood-cells ; (2) a diminution or change in the character of the red colouring-matter ; (3) a true scarcity of blood ; (4) an irregular distribution of the blood, there being more in the internal organs than at the surface of the body ; and (5) the character of the superficial layers of the skin, which are sometimes particularly rich in a deposit of yellow or dark pigment, thereby offering an obstruction to the passage of the light-rays emanating from the red blood-cells. There are families, and even races, who are possessed of a pale, tender, and yellowish skin, without being at all anaemic ; and it is necessary for the physician to determine whether the pale or yellowish complexion has developed grad ually, or whether it is peculiar to the family in question.
A transitory blanching of the skin is often seen in delicate and frail persons, who, especially during the winter, afford the impression of being anemic. In other cases nervous influences may serve to direct a greater amount of blood to the internal organs than goes to the skin. It is well known that persons suffering from cold hands and feet often complain of a rush of blood to the head or give evidences of abdominal congestion as shown by various digestive disturbances and hemorrhoidal conditions. Further more, it is a matter of common observation that anxmic individuals may suddenly show a bright red flush, which clearly proves that a pale com plexion need not necessarily be due to a lack of blood or insufficient activity of the red blood-cells, but may be caused by a faulty distribution of the fluid dependent upon nervous influences. When this condition is accompanied by vertigo, noises in the ears, and cardiac palpitation, it may be taken for granted that the symptoms are those of actual amemia, and that as the blood rushes to the surface of the body, the internal organs are more or less depleted of their proper quantity of blood. The flushing of the face in such cases is often falsely attributed to congestion of the brain, whereas the very opposite condition, an anaemia, is present. The following varieties of anxmia must he distinguished : (1) Cklorosis, which depends on the diminution to a greater or less extent of the colouring-matters of the blood. The name which is applied to this condition is derived from the Greek, and refers to the characteristic yellowish-green complexion of the afflicted individuals. See Plate XIV., Fig. 4.
(2) Ancemnia, which refers to an insufficiency of the essential components of the blood, —albumin, salts, etc. The watery constituents of the blood may be increased, however, so that the total quantity of fluid remains the same, or it may even be increased. This condition is termed hydrzemia or " watery blood." In the severer forms of this trouble, the total amount of the red colouring-matter may also be decreased, and on puncturing the skin an almost colourless drop of blood appears, which is only slightly sticky and coagulates with difficulty. This may be looked upon as due to a faulty admixture of the elements of the blood. It is not in all cases due to dietetic causes—that is, to unsuitable nourishment.