artificial production has as yet never been accomplished.

contains no iron.

Hematoidin

204. The fibrin thrown down when the blood coagulates may be demonstrated upon the slide in the form of very fine particles and filaments. A drop of blood is brought upon the slide and kept for a time in a moist chamber or on the table until it begins to clot; after which a cover-slip is applied and the preparation washed with water by continued irrigation. In this manner most of the red blood-corpuscles are removed. Lugol solution may now be added, which stains brown the filaments of the fibrin network adherent to the slide. In order to see the fibrin network in sections, it is better to use specimens previously fixed in alcohol; the sections are stained for ten minutes in a concentrated solution of gentian-violet in anilin water (Weigert), rinsed in normal salt solution, treated for about ten minutes with iodo-iodid of potassium solution, and then spread upon a slide and dried with filter-paper. They are now placed in a solution consisting of 2 parts of anilin oil and I part of xylol until they become perfectly transparent. This solution is then replaced by pure xylol and finally by Canada balsam. The fibrin network is stained a deep violet.

205. There are different methods and a variety of material at our disposal for the demonstration of the blood current through the vessels. The best object for this purpose is probably the frog. The procedure is as follows: The animal is immobilized by poisoning with curare. 1⁄2 gm. of a 1% aqueous solution injected into the dorsal lymph-sac will immobilize the frog in a short time. The exact dose can not, however, be given, as the commercial curare is not a uniform chemical compound; the dose must therefore be ascertained by experiment. As is well known, curare affects exclusively the nerve end-organs of striated voluntary muscle, but does not affect either the heart muscle or unstriated muscular tissue; hence the utility of curare for this purpose. In order to see the blood current, it is only necessary to stretch the transparent web between the frog's toes and fasten it with insect needles to a cork plate having a suitable opening. If the cork plate be large enough to accommodate the whole frog, it may be placed in such a position that its opening lies over that in the stage of the microscope. The web thus spread out may be examined with a medium magnification. The tongue of the frog is also used for the same purpose. As the latter is attached to the anterior angle of the lower jaw, it may be conveniently drawn out, suitably stretched, and then placed over the hole in the cork plate. A very good view of the circulation may be obtained by examining the mesentery of a frog. The migration of the leucocytes through the vessel-walls can also be studied in such preparations. An incision 0.5 cm. in length is made in the right axillary line through the skin of a frog (best in the male), care being taken not to injure any vessels (which can be seen through the skin in frogs possessing little pigment). The abdominal muscles are then incised and a pair of forceps introduced to grasp one of the presenting intestinal loops. The latter is then attached to the cork plate with needles, and the mesentery carefully stretched over the opening. On examining the specimen it is best to moisten it with normal salt solution and to cover the area to be examined with a fragment of a cover-glass. The lung may also be exam

ined, but here the incision must be farther forward.

206. To obtain a general idea of the structure of lymphatic glands,

sections are made of small glands fixed in alcohol or corrosive sublimate. They are then stained with hematoxylin and eosin. In such preparations the cortical and medullary substances can be studied; the trabeculæ and blood take the eosin stain.

207. The flattened endothelial cells covering the trabeculæ are brought to view by injecting a o. 1% solution of silver nitrate into a fresh lymphatic gland. After half an hour the gland is fixed with alcohol and carried through in the regular way; the sections should be quite thick (not under 20). After the sections have been mounted in Canada balsam and exposed to light for a short time, the endothelial mosaic will be seen wherever the silver nitrate has penetrated.

208. Fixing with Flemming's solution and staining with safranin is the best method for studying the germ centers of the lymph-follicles. Other fluids which bring out the mitoses may also be employed.

209. Reticular tissue is best demonstrated by sectioning a fresh gland with a freezing microtome, removing a section to a test-tube one-quarter filled with water, and agitating it. The lymphocytes are thus shaken out of the meshes of the reticulum, leaving the latter free for examination.

210. The same results can be obtained by placing a section prepared in the above-named manner upon a slide, wetting it with water, and carefully going over it with a camel's-hair brush. The lymphocytes adhere to the brush. Both methods (His, 61) may be applied to hardened sections which have lain in water for a day or so. In this case, however, the removal of the lymphocytes is not so easy as in fresh sections.

211. In thick sections the reticulum is hidden by the lymphocytes. If, on the other hand, very thin sections (not over 3μ) be made, especially of objects fixed in Flemming's solution, the adenoid reticulum stands out clearly without any further manipulation.

212. The reticular structure may also be demonstrated by an artificial digestion of the sections with trypsin. The sections are then agitated in water, spread on a slide, dried, then moistened with a picric acid solution (1 gm. in 15 c.c. of alcohol and 30 c.c. of water), again dried, covered with a few drops of fuchsin S solution (fuchsin S i gm., alcohol 33 c.c., water 66 c.c.), and left to stand for half an hour. The fuchsin solution is then carefully removed, the section washed again for a short time in the same picric acid solution, then treated with absolute alcohol, xylol, and finally mounted in Canada balsam. The reticular tissue of both lymphatic glands and spleen are stained a beautiful red (F. P. Mall).

213. The treatment of splenic tissue is practically the same as that of the lymphatic glands.

214. In all these organs (lymph-glands, spleen, and bone-marrow) a certain amount of fluid may be obtained by scraping the surface of the fresh tissue. This may then be examined in the same manner as blood and lymph (see Technic of same). Sections of lymph-glands and spleen previously fixed in alcohol, mercuric chlorid, or even in Flemming's solution may be examined by the granula methods of Ehrlich.

215. By using the chrome-silver method a peculiar network of reticular fibers may be seen in the spleen. (Gitterfasern; Oppel, 91.) 216. The examination of the bone-marrow belongs also to this chapThe marrow of the diaphysis is taken out by splitting the bone

ter.

longitudinally with a chisel. With a little practice, it is easy to obtain small pieces of the marrow, which are then fixed by the customary methods and cut into sections. In the epiphysis the examination is confined either to the pressing out of a small quantity of fluid with a vice, or to the decalcification of small masses of spongy bone, containing red bone-marrow. In the first case, methods applicable to blood examination are employed; in the second, section methods (see also the petrification method, T. 158) are used. The methods given for the preparation of lymph-glands and spleen are also applicable in many cases.

217. Enderlen has demonstrated reticular fibers (Gitterfasern) in the bone-marrow by means of the chrome-silver method.

TECHNIC (CIRCULATORY SYSTEM).

218. To obtain a topographical view of the layers composing the heart and vessels, sections are made of tissues that have been fixed in Müller's fluid, chromic acid, etc. If the specimens are to be studied in detail, small pieces must be used, and are best fixed in chromic-osmic mixtures or corrosive sublimate. Celloidin imbedding is recommended for general topographic work. The further treatment is elective.

219. The endothelium of the intima may be brought to view by silver nitrate impregnation methods, by injecting silver solutions into the vascular system, etc. (vid. T. 109). The endothelial elements of the smallest vessels and capillaries are then clearly defined by lines of silver. Larger vessels must be cut open, the intima separated, and pieces of its lamellæ examined.

220. With regard to the isolation of the muscle-cells of the myocardium and of the walls of the vessels, see T. 170 to 172.

221. Elastic elements, plates and networks are best observed in the tunica media of the vessels, very small pieces of which are treated for some hours with 33% potassium hydrate.

222. The appropriate stains for sectionwork are those which bring out the elastic elements and the smooth muscle-cells. For the former, orcein is used (vid. T. 138).

223. For demonstrating the distribution of the capillaries, the reader is referred to the injection methods (vid. T. 100 et seq.) The lymphcapillaries are injected by puncture (T. 111); compare also the methods of Altmann (T. 112).

III. THE DIGESTIVE ORGANS.

THE intestinal canal with the glands derived therefrom originates from the inner layer of the blastoderm, the entoderm. The latter, however, does not extend to the external openings of the body, as the ectoderm forms depressions at these points which grow inward toward the still imperforate fore and hind gut to communicate finally with its lumen. This applies as well to the formation of the primitive oral cavity, which is separated only secondarily into oral and nasal cavities, as to the anus. The anterior boundary

between the ectodermal and entodermal portions of the digestive tube consists of a plane passing through the opening of the posterior nares and continued downward along the palatopharyngeal arch. Everything lying anterior to this is of ectodermal origin, therefore the entire oral and nasal cavities with their derivatives. The lining of these cavities consists, however, of a true mucous membrane, closely resembling in its structure that of the intestinal tract.

A. THE ORAL CAVITY.

The epithelium of the oral mucous membrane is of the stratified squamous type, differing from the epithelium of the epidermis in that the stratum granulosum does not appear here as an independent layer. The stratum lucidum is also wanting, and the cornification of the layer analogous to the stratum corneum of the skin is not complete (compare Skin). In the mucous membrane the cells of even the most superficial layers contain nuclei, which, although partly atrophied, still show chromatin, and as a consequence are easily demonstrated.

Beneath the epithelium lies a tissue of mesodermic origin, also belonging to the mucous membrane and known as the mucosa or stratum proprium (lamina propria, tunica propria), in which numerous glands are situated. The mucosa consists of a fibrillar connective tissue with few elastic fibers, and of adenoid tissue containing numerous lymphoid cells; essentially, therefore, a diffusely distributed adenoid tissue with occasional lymph-follicles imbedded in its substance. The mucosa presents numerous papillæ, which are either simple or compound (branched) elevations of the mucosa, varying in length and density, according to their location and extending for variable distances into the overlying epithelium. As in the papillary layer of the corium (see Skin), so also here the superficial layer of the stratum proprium contains very fine elastic and connective-tissue elements which contribute to the structure of the papillæ. All these papillæ contain capillaries and arterioles which are derived from an arterial network in the mucosa. The lymphatics are similarly arranged.

At the red margin of the lips the papillæ are unusually high and are covered at their summits by a very thin epithelial layer (Fig. 173). Besides the sebaceous glands which lie at the angles of the mouth, and whose ducts open at the surface, there are here no other glandular structures. In the mucosa of the mucous membrane of the lips and cheeks the papillæ are low and broad; here also open the ducts of compound lobular, alveolar glands, the glandula labiales and buccales whose structure is similar to that of the large salivary glands (see these). The gums possess very long. and attenuated papillæ, covered by a very thin layer of epithelium, therefore bleeding at the slightest injury. That part of the gum

covering the tooth has no papillæ. The gums contain no glands. The papillæ of the hard palate are arranged obliquely, with their points directed toward the opening of the mouth. The papillæ of the soft palate are very low and may even be absent. They are somewhat higher on the anterior surface of the uvula. On the posterior surface of the latter occur ciliated epithelia distributed in islands between the areas of stratified squamous epithelium. In the soft palate and uvula are found small mucous glands.

Under the mucous membrane there is a layer consisting princi

Fig. 173.-Section through the lower lip of man; X 18.

pally of connective tissue and elastic fibers, the submucosa (stratum submucosum, tela submucosa). In the mucous membrane of the mouth the transition of the tissue of the mucosa into that of the submucosa is very gradual. The submucosa of the hard palate is closely connected with the periosteum and contains, especially at its posterior portion, numerous glands. In other regions of the mouth (lip) the glands extend also into the submucosa. The mucosa and epithelium lining the mouth cavity are richly supplied with nerves which terminate either in special sensory