Sections of blood discs are made by dipping a fine needle in a drop of blood as it exudes from a prick of the finger and drawing thin lines across the glass slip, allowing time to dry, and then cutting the lines across in all directions with a razor. The loosened portions should be removed with a camel’s-hair brush.
In birds, the blood discs are oval in shape and possess a nucleus, shown in [Fig. 425] B, in the blood of the fowl; this is rendered more apparent on adding a drop of acetic acid. The blood of fishes is also oval and nucleated, rather more pointed than that of birds. In reptiles generally the red blood discs are large, oval, nucleated bodies, the white corpuscles still preserving their invariable circular form and granular appearance. In the salamander and proteus the discs attain to their greatest size. In the former they measure 1⁄700th of an inch, and in the latter 1⁄400th.
Blood Crystals.—In addition to the elements described, the blood contains various crystalline forms, represented in [Fig. 425], C to H. In connection with the micro-spectroscope (p. 253), the spectra of certain blood crystals are given; although varying in different animals, sufficient uniformity prevails as to render them characteristic. The crystals are formed when a little blood is mixed with water on the slide, allowing a short time for crystallisation. Near the edge of the cover-glass, where crystals begin to form, they are more distinct, but a high power is required for their examination. In human blood the crystals are prismatic; in that of the guinea-pig, tetrahedral; in the blood of the mouse, octahedral. Other forms may be obtained by the aid of chemical reagents.
In human blood there are at least three distinct forms of crystals: Hæmatin is formed in normal blood, is made visible on the addition of a little water to blood, or by agitation with ether, so as to dissolve the cell-wall of the blood corpuscles, and allow the contents to escape. A drop of blood will furnish crystals large enough to be seen with a moderate power. Hæmatoidin crystals are abnormal products, found in connection with certain diseased conditions. These crystals are seen as represented at D. Hæmin crystals must be regarded as artificial chemical products, the result of treating blood with glacial acetic acid; the acicular crystals at E, reddish-brown in colour, are artificially produced.
Fig. 426.
1. White fibrous or non-elastic tissue; 2. Yellow fibrous elastic tissue.
Basement Membrane—Connective Tissue System.—Connective or areolar tissue is present almost throughout the whole of the human body, and serves to connect the various organs with one another, as well as to bind together the several parts. The muscles are surrounded by a connective tissue sheath; this penetrates into their substance, and binds together fasciculi and fibres. The same tissue is present in the skin and the mucous membranes; it also forms a sheath for the arteries, veins, and nerves. It is plentifully supplied by blood-vessels, and nerves pass through its substance. Microscopically, four different elements can be clearly made out:—1. Connective tissue cells or corpuscles; 2. White fibrous tissue; 3. Yellow fibrous tissue; 4. Ground substance.
On examining the connective tissue cells of young animals, various cells will be seen with fine granular contents, together with nuclei, lying in spaces in the ground substance, some branched, others flattened or rounded. Even tissues supposed to be homogeneous in structure, are on staining seen to have connective tissue cells, such as those represented in a section of the cornea of the eye (see p. 31). In this case the connective tissue cells are termed corneal corpuscles; the branched cells, it will be noticed, are united by branches.
The cells in the fibrous tissue of tendons are square or oblong, and form continuous rows. White fibrous tissue is distributed throughout the animal body, but in a variety of forms; it is found in the skin and other membranes, and in all parts where strength and flexibility are necessary. The structure of white and yellow fibrous tissues is shown in [Figs. 426] and [427].