BLOOD-BAIN. 192 BLOOD-VESSELS. Such organisms are often capable of impart ing to snow a pink, red, or bluish tint, and some- times exhibit the phenomenon of phosphores- cence. Similarly, black rain may be caused by the presence of a black fungus. The above- mentioned low forms of plants grow, especially in torrid zones, in ponds that hold water during a sliort season of each year, being dried up dur- ing seasons of drought which last for several months. From these desiccated basins the fine dust and minute fungi are raised into the at- mosphere by wind and carried for hundreds of miles. Decrease of temperature causes rainfall or snowfall, during which the dust-particles are carried to the earth. The gray dust, consisting principally of diatoms, being rather denser, may descend without rain. Such dust-showers have been observed to fall upon vessels in the Atlantic Ocean as far as a thousand miles west of the coast of Africa. Sliowers of blood-rain and dust are observed principally over the area lying east, west, and north of the Sahara, but they are not restricted to this region. See ^Kolian Accumu- lations. BLOODROOT. See Sanguinaria. BLOOD-STAINS. In some criminal trials it is a matter of extreme importance to determine vhetlicr certain stains found upon various weap- ons, upon clothing, or upon carpets, walls, furni- ture, etc., have been made by blood or by some otlior substance. Further, there comes np the question as to whether a blood-stain has been made by human blood or by the blood of one of the lower animals. The appearance of blood- stains varies, depending la'rgely on the character of the substance stained, age of the stain, etc. As a rule the more permeable the substance, the brighter is the stain. Thus, on cloth the serum is quickly absorbed, leaving a rather bright stain from the colored elements of the blood; while stains on hard surfaces, such as metal and pol- ished wood, are usually of a dark color. The question as to whether a given stain is blood is in most cases comparatively easy to answer. The determination may be made by chemical means, by the use of the microscope, or that of the spectroscope. The guaiacum test is as follows: If a drop of blood, or a drop of water with which a blood-stain has been moistened, be placed upon a blotting-paper and a drop of tincture of guaiacum added, the further addition of a drop of hydrogen peroxide will result in the produc- tion of a sapphire-blue color. The fact that cer- tain substances other than blood may give this or a similar reaction tends to vitiate the value of this test. Another test, known as the lioiiin test, is as follows: To a little of the powder scraped from a blood-stain is added a dro)) of glacial acetic acid with a trace of sodium chlo- ride. After standing for some time the mixture is gently heated. .s evaporation takes place the specimen should be examined under the micro- scope, wh.'n numerous rhomboidal crystals will be seen of a rcddisli, brown, or yellow color. These are i)rodu<'ed fiom the coloring matter of the blond, and are known as hemin crystals. A positive result from this test is extremely relia- ble. On the other hand, a failure to obtain crys- tals cannot be considered ])ositivc proof that the stain is not blood. Very accurate results may be obtained by spectroscopic examination of blood-stains. The fact that hemoglobin (from a fresh stain) and hematin (from an old stain) exist each in two states of oxidation, and that each state has its own characteristic absorption bands in the spectrum, increases the accuracy of this t^st. The microscope alone is often of value in determining the nature of blood-stains. Thus, if a stain be moistened with a little normal .saline solution (sodium chloride per cent, in water), and some of the fluid be examined under the microscope, it is often possible, even in the case of verj' old .stains, to recognize the blood-cor- puscles. The dilTerentiation of the blood of a man from that of a woman or child, or of the blood of one person from that of another, is apparently im- possible. The differentiation of a human blotid- stain from a stain made by blood from one of the lower animals is mentioned in the article Blood (q.v.). BLOODSTONE. See Heliotrope. BLOOD-TRANSFUSION. See Transfusion OF HlOOI). BLOOD-VESSELS. Tubes of various sizes which serve as channels for the passage of the blood to the different organs and tissues of the body. While all are connected with one another and with the heart, three main forms of vessels may be differentiated, depending on size, func- tion, and structure. These are capillaries, arte- ries, and veins. Capillaries. These are the smallest of the blood-vessels. They form extensive networks in almost all tissues and organs, and sen'e as con- nection betw'cen the arteries on the one hand and the veins on the other. Their average size is from 7 to 12 micromiljimeters. It is thus seen that they are little larger than the corpuscles which pass through them. The wall of a capil- lary consists of a single layer of endothelial cells. These cells are large and flat, with bulging nuclei, and are united to one another by cement sub- stance. This single layer of endothelium serves as the lining of the entire vascular system, ex- tending uninterruptedly throughout capillaries, arteries, veins, and heart. Arteries. Passing from a capillary toward the artery, the first change noted is the addition of a little connective tis.sue outside of the endo- thelium. This is the beginning of the outer coat of the artery, and is known as the tunica ailrcn- tilia, or the ailpcntitia. A few muscle-cells next appear between the connective tissue and the endothelium; and this small vessel with three coats, an outer connective tissue coat {adren- titiu), a middle coat consisting of a single layer of smooth nniscle-cells {tutiica media or media), and the lining endothelium (tunica intima or in- lima), is called an arteriole. In a larger artery these same coats are present, hut more complex. Thus the intima, instead of being a single layer of endothelium, consists of three layers — an in- nei' eiulcitliclial, a middle layer of connective tis- sue, and a limiting elastic membrane". The nw- dill or musfular coat consists of a thick layer of smooth inusele-cells, ])laced circularly, while the udrcniitia layer is also of considerable thickness. The aorta differs from other arteries in having a much larger amount of elastic tissue in its walls. This elastic tissue is distributed through- out the muscular coat, almost equaling the nnis- cle-lissue itself in quantity. There is usually no definite elastic membrane limiting the intima. Veins. These have the same coats as do the arteries. Their walls are, however, thinner and
