Page:1902 Encyclopædia Britannica - Volume 25 - A-AUS.pdf/438

 392

ANATOMY

man lias been scrutinized with, the greatest minuteness by hundreds of workers, and knowledge of its details and of its variations is far more extensive and more accurate than that of the body of any other animal. The methods of anatomical study are of two sorts— those whereby the structures are unravelled as far as it is possible to discriminate them by the naked eye, Methods. and tjlose w]ierePy the finer texture of the tissues is displayed and examined by the aid of the microscope. The procedures for either microscopic. or macroscopic study are twofold—dissection and the cutting of sections. The former* is the art of separating the component parts, and of rendering them available for study by the removal of the connective tissues and fat which envelop them, while by the latter the relative anatomy of the parts can be best exhibited. Macroscopic sections are made of the whole body or limb, by freezing and cutting the sections with a fine saw, or by hardening the parts by the injection of formalin, and cutting with a large and sharp knife (Fig. 1). The topography of regions can

Fig. 1.—Section through the thorax on the level of the nipple, showing the position of the heart and lungs. (Merkel.) be most satisfactorily made out by the examination and comparison of successive sections made at short distances from each other. Sections for microscopic examination are most easily made of small parts of organs or whole organs hardened and embedded in some supporting material, such as paraffin wax or celloidin. These are cut into transparent films by means of a section-cutter, various patterns of which are in use. The best of these have an adjustment by means of which the sections can be cut of any required thickness, and the successive sections are preserved in the order in which they are cut. (The works of Braune, Rudinger, and Symington give excellent plates of macroscopic sections of the body.) In preparing parts for dissection, it is of advantage that the tubular systems should be filled with some injected material, such as paint, wax, or starch. (Details of the modern methods in use are given by Gronroos, Anat. Anzeiger, 1898.) Microscopic sections are usually stained with certain soluble pigments, such as carmine, logwood, or eosin, which have the property of differentiating special tissues, such as protoplasm, cartilage, muscle, &c., colouring some and leaving others unstained. In modern anatomical technology, the number of such stains in use is large. (These methods of preparation are described in the text-books of histology by Schafer Stirling, Bolles Lee, &c.) In recent years, great progress has been made in the knowledge of the details of human structure. Many parts, such as the convolutions of the brain, the folds of the intestine, &c., formerly supposed to be indefinitely variable as if they were arranged at haphazard, have been shown to be built up, even in their most minute

details, in a definite manner capable of formulated description ; indeed, it is probable that no structure is so completely variable that its different conditions are incapable of reduction to law. (Of modern text-books, those of Bardeleben and Gegenbaur in German, of Poirier and Testut in French, are the best; English text-books, such as Gray, Morris and Gerrish are excellent from the descriptive side, but almost completely ignore morphology. Quain’s work in its latest edition is of the nature of a compromise between the morphological and the descriptive.) Ultimate Analysis of the Body. The human body ultimately consists of form-elements of three sorts—cells, intercellular substance, and fibres. In its earliest form the whole organism was a cells single cell, and the first processes of growth consist in the multiplication of cells by division. A cell

Fig. 2.—The cell and its process of division. A, Cell showing the network of spongioplasm ; c, centrosome; w, nucleus ; n', nucleolus. B, First stage of division formation of chromatin wreath. C, Monaster stage with spindle and centrosomes at the poles. D, Chromatin star of this stage, seen from above. E, Formation of diaster. F, The beginning of division and of the reconstruction of the nuclei. G, Stage of division nearly completed. is not a homogeneous speck of protoplasm, but is a specialized structure exhibiting a comparatively complex organization (Fig. 2). The chief mass of the cell substance consists of a cytomiton, or network of protoplasmic threads, each of which often contains granules of some substance derived from the metabolism of protoplasm, the meshes of which are filled with a more fluid material or enchylema. The nucleus also is complex and enclosed by an apparently structureless limiting membrane, within which is a network of at least two kinds of material. One portion is capable of being stained by carmine, and on that account is called chromatin. The other part of the karyomiton or nuclear net is incapable of being stained. Although