No. 1. Pavement epithelium, taken from an internal membrane; 2. Columnar epithelium, from the intestine of a rabbit, showing central fat globules, and at str a fine ciliated border; 3. A so-called “goblet”-cell.
Spheroidal epithelium is confined to the closed cavities of the body, and in the internal structure of the ducts of secreting glands. The cells are, for the most part, circular, although some are flattened out at the sides in which they are in contact with each other ([Plate XIX]., No. 1a). Specimens of this form may be taken from the internal surface of one of the lower animals with a scalpel. The collected matter must be placed in a drop of distilled water and examined with a high power.
Ciliated epithelium is characterised by the presence of those fine hair-like filaments (cilia) attached to the free surface of the cell. During life, and for some time after death, the cilia are seen to retain their constant waving motion. The cilia all move in one direction and rhythmically, thus giving rise to the appearance of a succession of undulations. Ciliated epithelium is found lining the mucous membrane of the air passages and nasal ducts, and wherever it is necessary to urge on a secretion by mechanical means, ciliated epithelium exists. Specimens for examination are easily obtained from the oyster, and with care will show the characteristic motion. A portion of a gill separated from the mollusc will live on for a considerable time if kept in a little of its natural secretion. The parameciæ, rotifera, and all the ciliata, are furnished with cilia as a means of locomotion and obtaining sustenance. By snipping off a small piece from the gills of the mussel, always accessible to the microscopist, and covering it over with thin glass to prevent evaporation of the animal juices, its cilia will continue to work for hours.
Lymph and Blood, [Fig. 425] B, a a.—There are other cells in the animal body which possess a certain amount of resemblance to those confined to the more superficial structures—i.e., the lymph, chyle, and blood. These fluids present in one respect a physical uniformity of composition, and a resemblance in the size of their characteristic corpuscles. Chyle contains besides the corpuscles of lymph, a quantity of minute granules which imparts a white colour to the fluid. Intermixed are oil globules, free nuclei, and sometimes a few red blood discs. Chyle may be had for microscopic examination by squeezing a little juice from the lymphatic gland of a sheep just slaughtered.
Fig. 425.—Human Blood Corpuscles and Crystals.
A. a a. Red blood corpuscles lying flat on the warm stage; b b. in profile; c c. arranged in rouleaux; d. crenated; e. rendered spherical by water; I. leucocytes and white amœboid corpuscles; B. Blood discs of fowl, red and white, others seen in convexity and with a nucleus. Blood Crystals.—C. Hæmatin from human blood; D. Hæmatoidin; E. Hæmin; F. Tetrahedral; G. Pentagonal; H. Octahedral crystals from blood of mouse.
Blood Corpuscles or cells vary considerably in mammals, birds, reptiles, and fishes. [Fig. 102] (page 143) is a microphotograph of a drop of blood magnified 3,500 times; and [Fig. 425], A, shows both red and white discs drawn to scale, magnified 1,200 diameters. The red corpuscles of human blood are distinguished by their clearly defined outlines and dark centres. Each disc is biconcave in form, and hence the whole surface cannot be focussed at the same time. When the circumference is well illuminated the centre is dark, but by bringing the objective nearer to the object, the concavity of the disc is brought into focus. It generally happens that blood corpuscles, on being first drawn, run together, and present the appearance of rolls of coins; or they may be scattered about over the field. There is a considerable difference in the form of the discs; they are circular in all mammals, except the camel, dromedary, and llama, these being oval. In profile blood corpuscles are biconcave, their investing membrane is homogeneous and elastic, and will readily move along the smallest capillary vessels. There is no trace of a nucleus in the blood-discs of the adult Mammalia, while in size they bear no proportion to the bulk of the animal in whose blood-vessels they circulate. The corpuscles of Mammalia in general are like those of man in form and size, being either a little larger or smaller. The most marked exception is the blood of the musk-deer, in which the corpuscles are of extreme smallness, about the 1⁄12000th of an inch in diameter. In the elephant they are large, about 1⁄2700th of an inch in diameter. The goat, among common animals, has very small corpuscles, but they are, withal, twice as large as those of the musk-deer. In the Menobranchus lateralis they are of a much larger size than in any animal, being the 1⁄350th of an inch; in the proteus, the 1⁄400th of an inch in the longest diameter; in the salamander, or water-newt, 1⁄600th; in the frog, 1⁄900th; lizards, 1⁄1400th; in birds, 1⁄1700th; and in man, 1⁄3200th of an inch. Of fishes, the cartilaginous have the largest corpuscles; in gold-fish, they are about the 1⁄1700th of an inch in their longest diameter.
The large size of the blood discs in reptiles, especially in the Batrachia, has been of great service to physiologists by enabling them to ascertain many particulars regarding structure which could not have been otherwise determined with certainty. The value of the spectroscope in the chemical examination of the blood has been already referred to. See page 252.
White corpuscles or leucocytes ([Fig. 425], I) differ materially from the red. They are large, spheroidal, finely granular masses of about 1⁄2800th of an inch in diameter. In a cubic millimètre of human blood there are about 10,000 white corpuscles. They have a lower specific gravity than the red, have no cell-wall, and their substance mainly consists of protoplasm. The internal granular appearance is now believed to be due to a fine intercellular network having small dots at the intersections of the web. In the meshes of the net a hyaline substance is interspersed. They possess one or more nuclei; these are seen on the application of a few drops of acetic acid. When examined in a perfectly fresh state, especially if the glass slide be placed on the warm stage of the microscope, they exhibit a spontaneous change of shape, amœba-like, such movements being accordingly termed amœboid. The movements referred to consist in the protrusion of processes of protoplasm which are retracted and other processes protruded as represented ([Fig. 425], I). Both in human blood and in newts there are colourless corpuscles which contain coarser granules than others; these are called granular corpuscles. Some are shown near the amœboid bodies. The white corpuscles are readily found in various tissues of the body, as in the lymphatic glands. In inflammatory diseases these leucocytes pass through the walls of the capillaries into the tissues, and form morbid products, pus-cells.