Text-book of forensic medicine and toxicology

by R. J. M. Buchanan

Fig. 7.—Photo-micrograph of cotton fibres, × 250.
(R. J. M. Buchanan.)

In examining a clasp-knife or any hinged weapon for blood-stains, the instrument should be taken to pieces and all the hinges and recesses carefully examined, for in these places blood may be found, although the weapon had previously been wiped clean, and appear free from stains.

5. On Leather.—The tannic acid in leather forms a compound with blood which is insoluble in the solvents generally used. A thin shaving of the stained portion should be taken and folded, with the stained surface outwards, in the form of a loop. If the outer surface of the loop with the stain be made to touch the surface of the glycerine and water solution, at the same time taking care that the leather itself be not moistened, a recent stain may yield a sufficient quantity of colouring matter for the purposes of examination. Failing this, the shaving should be digested in a small quantity of a two per cent. solution of hydrochloric acid in distilled water (Sorby).

Microscopical Examination

The microscopical examination of blood-stains, for the purpose of identifying the presence of the red blood corpuscles, is especially applicable to recent stains. In these the corpuscles may retain, to a great extent, their normal characters; but their condition varies with the age of the stain; they become altered in appearance and irregular in shape with increasing age, until a stage is reached when they become completely disintegrated and unrecognisable. Having obtained a solution of a stain by one of the methods recommended, a few drops should be placed upon a clean microscope slide and covered with a No. 1 cover-glass. In a recent stain, where minute coagula are present, one may be placed on a microscope slide and moistened by breathing upon it several times, and then covering it with a No. 1 slip, or a drop of the glycerine solution may be allowed to act upon it on the slide until it be sufficiently moistened, when it should be covered in the same way. The preparation should be examined through the microscope with a good lens (preferably a ¹/₁₂th oil immersion), magnifying 300 to 400 diameters, and if any corpuscles be found, their characters should be carefully observed and noted.

All such specimens should be carefully preserved and labelled with a description of the method of preparation, the case to which they belong, the date of preparation, and the signature of the individual who has made and examined them. It is essential that the preparation and examination of the specimen should be made by the same individual. In certain cases the conditions may be sufficiently favourable to allow of the production of stained specimens, which can be mounted so as to retain their original characters permanently. In every case it is advisable to pursue the investigation with this object in view.

This process is especially applicable to recent blood-stains, in which, from preliminary examination, the presence of blood corpuscles has been determined; where complete disintegration of the blood corpuscles has taken place it would not be of any value.

It may so happen that by means of stained specimens the identity of blood corpuscles may be more easily established, when the result of examination is uncertain in a specimen not so prepared.

By the action of certain dyes upon the corpuscles their special features are rendered easier of recognition. Any of the approved methods of preparing blood films for general clinical purposes, which will suit the circumstances, may be employed. An easy and reliable method is as follows. A drop of the solution of the blood-stain properly prepared as previously recommended, or if obtainable a small coagulum moistened with normal saline solution, is placed on a clean coverslip and spread evenly over its surface with the aid of a fine glass rod. The film is allowed to dry in the air, covered with a watch-glass for protection against dust. When dry it is passed three times through the flame of a Bunsen burner, or placed in a mixture of equal parts of absolute alcohol and ether, to fix it. After fixation it should be placed for a minute or more in an aqueous solution of eosin.