Common objects of the microscope

by J. G. Wood

Fig. [17] is a part of the shaft of a young feather taken from the canary, given for the purpose of showing the form of the cells of which the pith is composed. Fig. [20] is part of the down from a sparrow’s feather, showing its peculiar structure; and Fig. [21] is a portion of one of the long drooping feathers of the cock’s tail.

Fig. [13] exhibits a transverse section of one of the large hairs or spines from the hedgehog, and shows the disposition of the firm, horn-like exterior, and the arrangement of the cells. Sections of various kinds of hair are interesting objects, and are easily made by tying a bundle of them together, soaking them in gum, hardening in spirit, and then cutting thin slices with a razor. A little glycerine will dissolve the gum, and the sections of hair will be well shown. Unless some such precaution be taken, the elasticity of the hair will cause the tiny sections to fly in all directions, and there will be no hope of recovering them.

Several examples of the skin are also given. Fig. [27] is a section through the skin of the human finger, including the whole of one of the little ridges which are seen upon the extremity of every finger, and half of two others. The cuticle, epidermis, or scarf-skin, as it is indifferently termed, is formed of cells or scales, much flattened and horny in the upper layers, rounder and plumper below. The true skin, or “cutis,” is fibrous in structure, and lies immediately beneath, the two together constituting the skin, properly so called. Beneath lies a layer of tissue filled with fatty globules, and containing the glands by which the perspiration is secreted.

One of the tubes or channels by which these glands are enabled to pour their contents to the outside of the body, and, if they be kept perfectly clean, to disperse them into the air, is seen running up the centre of the figure, and terminating in a cup-shaped orifice on the surface of the cuticle. On the palm of the hand very nearly three thousand of these ducts lie within the compass of a square inch, and more than a thousand in every square inch of the arm and other portions of the body, so that the multitude of these valuable organs may be well estimated, together with the absolute necessity for keeping the skin perfectly clean in order to enjoy full health.

Fig. [1] shows a specimen of epidermis taken from the skin of a frog, exhibiting the flattened cells which constitute that structure, and the oval or slightly elongated nuclei, of which each cell has one. In Fig. [32], a portion of a bat’s wing, the arrangement of the pigment is remarkably pretty. Immediately above, at Fig. [31], is some of the pigment taken from the back of the human eye-ball. The shape of the pigment cells is well shown. Similar specimens may easily be obtained from the back of a sheep’s eye which has been hardened in spirit, or from that of a boiled fish. Fig. [33] shows the pigment in the shell of the prawn.

On various parts of animal structures, such as the lining of internal cavities, the interior of the mouth, and other similar portions of the body, the cells are developed into a special form, which is called “Epithélium,” and which corresponds to the epidermis of the exterior surface of the body. The cells which form this substance are of different shapes, according to their locality. On the tongue, for example (for which see Fig. [11]), they are flattened, and exhibit their nucleus, in which the nucléolus may be discovered with a little care. Cells of this kind are rounded, as in the case just mentioned, or angular, and in either case they are termed squamous (i.e., scaly) epithelium. Sometimes they are like a number of cylinders, cones, or pyramids, ranged closely together, and are then called cylindrical epithelium. Sometimes the free ends of cylindrical epithelium are furnished with a number of vibrating filaments or cilia, and in this case the structure is called “ciliated” epithelium. Cylindrical epithelium may be found in the ducts of the glands which open into the intestines, as well as in the glands that secrete tears; and ciliated epithelium is seen largely in the windpipe, the interior of the nose, etc. A specimen taken from the nose is seen at Fig. [15]. A beautiful example of ciliated epithelium is to be found in the gills of the mussel. A portion of one of the yellowish bands which lie along the edge of the shell on the opening side is carefully removed with sharp scissors, and examined in the shell-liquor, being protected from pressure by placing a piece of paper beneath each end of the cover-glass. Such a preparation is shown in Plate IX. Fig. [39], but no drawing can give an idea of its wonderful beauty and interest. The cilia will continue to move for a long time after removal from the shell.

Bone in its various stages is figured on Plate X.

Fig. [9] is a good example of human bone, and is a thin transverse section taken from the thigh. When cut across, bone exhibits a whitish structure filled with little dottings that become more numerous towards the centre, and are almost invisible towards the circumference. In the centre of the bone there is a cavity, which contains marrow in the mammalia and air in the birds. When placed under a microscope, bone presents the appearance shown in the illustration.

The large aperture in the centre is one of innumerable tubes that run along the bone, and serve to allow a passage to the vessels which convey blood from one part of the bone to another. They are technically called Haversian canals, and if a longitudinal section be made they will be found running tolerably parallel, and communicating freely with each other. Around each Haversian canal may be seen a number of little black spots with lines radiating in all directions, and looking something like flattened insects. These are termed bone-cells or “lacúnæ,” and the little black lines are called “canalículi.” In the living state they contain cells which are concerned in the growth of the bone, and these may be made evident by softening fresh bone with acid, cutting sections of it, and staining. When viewed by transmitted light the lacunæ and canaliculi are black; but when seen by dark-field illumination the Haversian canals become black, and the lacunæ are white.