TISSUES AND ORGANS. The examination of pieces of tissue or of organs for the purpose of determining the nature of the disease affecting them is often of great importance.
Some tissues may be examined in the fresh state by simply teasing apart in such an inert fluid as normal saline solution (three-quarters per cent. aqueous solution of sodium chloride). The satisfactory examination of most tissues re quires, however, a more or less elaborate prelimi nary preparation. This consists in (1) fixing, (2) hardening, (3) imbedding, (4) section cut ting, (5) staining, and (6) mounting.
(1) Fising.—This consists in placing the tis sue, as soon as possible after its removal, in a solution which will kill the tissue elements rap idly so that they retain the same form and structure that they had during life. Of the most commonly used fixing agents may be men tioned alcohol; formalin, in aqueous solutions of from to 10 per cent.; and Miiller's fluid (potassium dichromate, 2.5 grams; sodium sul phate, 1 gram; water, 100 c. c.).
(2) Hardening and Preserving.—After fixing, tissues are usually thoroughly washed in run ning water and then hardened in graded alcohols, i.e. first in 50 per cent., then in 60 per cent., then in SO per cent. For permanent preservation they are usually left in SO per cent. alcohol.
(3) Imbedding.—This is for the purpose of impregnating the tissues with some substance which \ 11 I hold them together during the subse quent manipulations. The now most commonly employed imbedding mass is celloidin, although for special purposes paraffin is used.
(4) Section Cutting.—This is now accom plished by means of an instrument known as a microtome. While many of these instruments are quite complicated, the purpose of them all is to carry a knife through the specimen in such a way that sections of any desired thickness may be obtained.
(5) Staining.—Sections may he stained in a great variety of ways. For genera] purposes what is known as 'staining double' gives satis factory pictures. This is accomplished by stain ing the specimen first in a watery solution of haemotoxylin and then in an alcoholic solution of eosin. The specimens are next placed in oil of origanum, which removes the alcohol and ren ders the sections more transparent (`clearing').
(6) Mounting.—From the oil the section is transferred to a glass slide, the excess of oil re moved by blotting with filter paper, a drop of Canada balsam placed on the specimen, and the whole covered by means of a cover glass. This makes a permanent mount.
For other methods of staining and mounting the reader is referred to special text-books on histology and histological technique.
BIBLIOGRAPHY. Carpenter, The ilicroseopc and Bibliography. Carpenter, The ilicroseopc and Its Revelations (Sth ed., Philadelphia, 1901) ; Lee, The Microtomist's Vade-ilecum (5th ed., Philadelphia, 1900) ; Ilenneguy, Methades techniques de Panatomie rnicroscopique (Paris, 1887) ; Szymonowiez, Lehrbuch der Histologic (Wfirzburg, 1900) ; Dunham, Histology, Normal and Morbid jPhiladelphia, 189S) ; Clarkson, A Text-Book of Histology (Philadelphia, 1896) ; Mimi-Davidoff-Huber, Lehrbuch der Histologic des Menschen (Wiesbaden, 1903) ; Star, His tologie (10th ed., Jena, 1903) ; Abbott, Princi ples of Bacteriology (3d ed., Philadelphia, ]895) ; Delafield and Prudden, Pathological Anatomy and Histology (6th ed., New York, 1901) ; Nich ols, Clinical Laboratory Methods (New York, 1902) ; Peyer, Atlas on Clinical Microscopy (New York, 1885) ; I'ellew, Manual of Chemis try (New York, 1892).