tractility, in the healthy body, and in greater numbers in diseased states; in some cases re-entering the lymphatic circulation, and in others penetrating into various tissues. The pus-corpuscles appearing in the vicinity of inflamed parts are shown by this discovery, made by Waller and Cohnheim, to be nothing but migratory lymphoid or white cells of the blood. The change of form and place of these amoeboid cells is readily seen by placing a drop of frog's blood on a glass cover, and inverting it over a moist cell. As it coagulates, a zone of serum extends round the clot, in which the migrated cells will be found. The colorless cells originate in the chyle and lymphsystems, although some may come from the spleen and the medulla of bones, multiplying in the blood itself, and they pass into red corpuscles. Transitional forms have been found in the general mass of blood, in the spleen, and in the marrow of bones. The white or colorless cells of blood are identical with the cells of chyle, lymph, pus, mucus, and saliva. They are often described under the term leucocytes (white cells.) The leucocytes of saliva (salivary corpuscles) and of pus contain granules or globules of formed material, which exhibit for some time a peculiar dancing movement (see page 120). When at rest, or in a lifeless condition, the white cells are of spheroidal form, and generally exhibit granules and globules of fat. Acetic acid develops a nucleus, and sometimes splits it into several (Plate XVIII, Fig. 140). II. Lymph and Chyle. The vessels of the lymphatic or absorbent system receive the liquid part of the blood which has passed from the capillaries, together with the products of decomposition in the tissues, and return them to the circulation. The lymphatics of the intestinal canal receive during digestion a mixture of albuminous and fatty matters, which is known as chyle, and these vessels have obtained the name of lacteals. The cells in this fluid are leucocytes, identical with white cells in blood. They originate in the lymphatic glands and "Peyer's patches" of the intestine, and are the corpuscles of these organs which have been carried off by the fluid stream. III. Mucus. Is a tenacious semifluid substance which covers the surface of mucous membranes. It contains cast-off epithelial and gland-cells, and the mucus corpuscle, which, as we have before said, is identical with other leucocytes. Synovial fluid is of similar nature. It is now regarded as a transformation product of the epithelial cells, and not to originate as a secretion from special glands (Plate XVIII, Fig. 141). 2. EPITHELIUM AND ITS APPENDAGES. Epithelium (from ε, upon, and aw, to sprout) is so called since it was formerly supposed to sprout from membrane. It is a tissue formed of cells more or less closely associated, which is found in layers upon external and internal surfaces. The cells are generally transparent, with vesicular, homogeneous, or granular nuclei, the latter being the remains of the original leucocyte or bioplast. In the older cells the nucleus is absent, the entire mass having been transformed. The forms of epithelial cells vary according to situation or function. The original form is spheroidal, but changes by compression, etc. 1. Tessellated or pavement epithelium (Plate XVIII, a, Fig. 142). These are cells whose formed material is flattened, and which are united at their edges. They are sometimes hexagonal, and often polyhedral, in form. Examples: Serous and synovial membranes; the pos terior layer of the cornea; the peritoneal surface; the interior of blood vessels, and shut sacs generally. 2. Squamous or scaly epithelium. The cells are flat, and overlap each other at the edges (Plate XVIII, b, Fig. 142). Examples: Epidermis; many parts of mucous membranes, as the mouth, fundus of bladder, vagina, etc. 3. Glandular epithelium (Plate XVIII, c, Fig. 142). The cells are round or oval bioplasts, often polyhedral from pressure, and the formed material is often soft. Examples: Liver cells, convoluted tubes of kidney, and interior of glands generally. 4. Columnar epithelium (Plate XVIII, d, Fig. 142). Cells cylindrical or oblong, arranged side by side. A bird's-eye view shows them similar to the tessellated form, hence they should be seen from the side. Examples: Villi and follicles of intestine, ducts of glands, urethra, etc. Some of the columnar or cylinder-cells have a thickened border or lid perforated with minute pores (Plate XVIII, f, Fig. 142). They are found in the small intestine, gallbladder, and biliary ducts. 5. Ciliated epithelium (Plate XVIII, e, Fig. 142). These are cylindrical cells having vibratile cilia, whose motions produce a current in the surrounding fluid. Examples: The upper and back nasal passages, the pharynx, bronchi, Fallopian tubes, etc. The Hair.-Hairs are filiform appendages, composed of a modified epithelial tissue of rather complex structure. They originate in a follicle, which is a folding in of the skin. The shaft of the hair is the portion projecting above the skin, and the root is concealed in the hair-follicle. The bulb of the root is the rounded terminal part, which is hollow below, and rests on a papilla which rises from the floor of the follicle (Plate XIX, Fig. 143). Between the follicle and hair is a sheath, which is divided |