TABLE OF BLOOD-CORPUSCLES (Diameters in fractions. The red blood-corpuscles of man and the mammalia generally can be readily distinguished, however, from those of birds, reptiles, and fishes, the latter being not only relatively of large size, but oval in form and nucleated (Fig. 16). It should be mentioned, in this connection, that the red corpuscles of the camel and llama are oval, and that those of the lamprey are somewhat circular. No difficulty should arise on account of this circumstance, for, as in the case of the camelidæ, the corpuscles are without a nucleus, though ovoid in form; while in the lamprey the corpuscles are nucleated, even if circular. The fact that the red corpuscle of a bird differs in form and in size from that of a mammal is important from a medico-legal point of view. As an illustration, let us suppose that in the case of blood-stains found in a wagon in which a person was suspected to have been murdered, the explanation offered by the person accused of the crime was that the blood spilled was that of a chicken which had been killed by him for market. If, after examination, the expert testified that the red blood-corpuscles obtained from the blood were round and without nucleus, that testimony alone would prove that the statement of the defendant was false, and would be presumptive evidence of guilt, and might lead to the confession of the crime and to conviction. Blood-crystals (Fig. 17), due to the crystallization of the hæmoglobin or coloring matter of the blood, constitute an important proof of the existence of blood.1 The blood-crystals can be readily obtained from freshly-drawn human blood by evaporating a drop of the blood to dryness on a glass slide, adding a drop of distilled water, and allowing the water to evaporate under a thin glass cover. The glass slide having been transferred to the stage of the microscope, the crystals will soon appear in certain forms 1 Preyer: Die Blut-Crystalle, Jena, 1871; Dragendorf in Maschka, vol. i. S. 483. and sizes, but usually as small prisms.' If the blood submitted for examination has, however, undergone changes such as would have occurred in the case of a blood-clot, the following will be found a convenient method of obtaining the crystals: Triturate the substances suspected to be blood in a mortar with a little common salt, add glacial acetic acid, and warm the mixture till bubbles appear; then set aside to cool. If the substance so treated contains hæmatin, that is, modified hæmoglobin, crystals of hæmatin hydrochlorate will appear as rhombic tablets, stars, or crosses. The presence of such blood-crystals may be regarded as proving that the material from which they were obtained was blood, but not necessarily human blood, since the crystals of the blood of certain animals are undistinguishable from those of man. Therefore, the presence of blood-crystals as evidence that a suspected material from which they have been obtained is blood is even of less value from a medico-legal point of view than the presence of blood-corpuscles, since the microscopist is unable, from the form of the latter at least, to distinguish the blood of the mammalia from that of other vertebrata. The spectroscopic method of investigating blood-stains is based upon the fact that blood interferes with the transmission of certain rays of light, and that it gives rise to what are known as the dark absorption bands of the blood spectrum. It is well known that when sunlight is transmitted through a prism it is decomposed into the seven colors: Violet, indigo, blue, green, yellow, orange, red. If, however, a weak solution of arterial blood be placed 1 Another method is to mix together in a watch-glass a drop of blood with glacial acetic acid in excess, and evaporate to dryness slowly over a spirit lamp. The mass so obtained, when viewed under the microscope, will usually exhibit the crystals in great numbers. |