affecting the brain that the ganglion cells may be quite easily isolated. To effect this a very small piece of the brain-calf's brain is perhaps the best is placed in fifty or sixty times its volume of the solution for twenty-four hours, and then carefully teased out under a good dissecting microscope.

Both weak chromic acid and alcohol may be used for isolating muscular fibres. Flies and beetles are perhaps the best for this purpose. The muscles of the wings (not those of the legs) should be torn out with fine forceps, and little bits, the smaller the better, placed in thirty per cent. spirit for twenty-four hours, and then dissociated or pulled apart on a glass slide, with fine needles. With sufficient care it is possible to separate the single fibrillæ of each fibre, and when stained with hematoxiline the

(FIG. 75.) ISOLATED MUSCULAR FIBRE OF COMMON WATER BEETLE.

alternating lines, dark and light (Figure 75), appear very sharply. These lines are those that make the muscles transversely striated. The cause of this striated appearance is not yet fully determined, but it is apparently connected with greater perfection of the muscular fibre than is found in the unstriated form. Different as is muscle in appearance from cells yet it originates from them, and is in fact formed of metamorphosed cells, by a series of changes all as great as those which produce bone.

We have still to notice a very important class of procedures, namely, injections. In the higher animals we find two distinct sets of vessels ramifying through the whole body: one of these is the system of blood-vessels, the other the lymphatic system. As is well known to all, the blood-vessels branch out into very fine tubes that form a complicated net-work in every part of the body, so fine that it can only be followed when the tubes or capillaries have been artificially filled with a colored matter. The same is true of the lymph-vessels, but to an even greater extent. Many of the structures of the body are permeated by connective tissue, and in this tissue there are numerous cavities filled with fluid; they are in communication with very delicate tubes, the lymphatic capillaries, which soon unite into larger canals, and these form branches which gradually join together and lead to the thoracic duct or main stem, which empties into the veins just before they open into the heart. The branches of this tubular system are provided with valves so arranged that the liquid contained in the tubes can only pass upward or towards the main stem. Now when any

motion takes place, some of the liquid in the cavities of the connective tissue is pressed into the lymphatics and so slowly driven onwards into the heart. To counterbalance this loss of intercellular fluid, certain constituents of the blood exude through the walls of the capillaries and keep up the supply. There is, therefore, a double circulation: one within the blood-vessels, and another from the walls of the capillaries through the lymphatics. The liquid in both circulations is ultimately returned to the heart.

Different methods have to be employed for injecting the two systems. In the case of the blood-vessels a rather large syringe may be used, provided with a point small enough to pass into the artery of the part to be injected. The artery should be carefully laid bare and cut about half way through; the point of the syringe, which must be previously filled with the injection mass, is pushed into the artery and firmly tied in place. In many cases it is quite sufficient to inject a cold saturated solution of Prussian blue in water, or when more perfect preparations are wanted, a little gelatine may be added; in this case, however, there arises the inconvenience that both the injection mass and the organ to be injected have to be kept warm while the operation is going on, otherwise the gelatine solidifies.

To prepare a "warm" injection mass, the following method is, perhaps, the best. A solution of Prussian blue is necessary; this the histologist must make for himself. To do this take a concentrated solution of sulphate of protoxide of iron in distilled water, and pour it slowly into a concentrated solution of yellow prussiate of potassium; a precipitate of insoluble Prussian blue is formed. There should be a small excess of prussiate at the end of the operation, to test which take out a drop and add to it a little of the sulphate. If there is any free prussiate still present, a blue precipitate is thrown down. Filter through a felt strainer, below which a funnel with a paper filter has been placed. Pour water on to the strainer in small quantities at a time, and continue filtering; this operation must be kept up for several days, until the liquid below the second filter appears distinctly blue. The matter on the felt strainer is then removed and dissolved in distilled water. This solution is admirable for cold injections or for filling the lymph-vessels, as will be described presently. There should always remain an excess of blue in the vessel in order to be sure that the solution is saturated; as the solution is removed it may be replaced by dis

tilled water, as long as there is any blue left. To make the "warm" injection mass take twenty-five parts of the Prussianblue solution and one part gelatine. The latter must be of the finest quality, as otherwise it produces a granular precipitate which renders it useless for histological purposes. Put the gelatine to soak for half an hour in distilled water, then remove and wash it; place it in a glass vessel and warm it in a water-bath, when it will melt in the water it has absorbed. The Prussian blue is put in another vessel in the same water-bath, so that the two liquids are at the same temperature. Pour the gelatine, little by little, into the blue, stirring constantly with a glass rod. Keep on warming and stirring until the granular precipitate formed at first disappears. Upon being filtered through a piece of clean flannel, the mass is ready for use.

It requires only to be slightly warmed to become liquid, and the organ to be injected does not need to be heated to so high a temperature as is necessary in using many other injection masses; there is, therefore, no danger of injuring the tissues by subjecting them to too high a temperature. The injection should be continued until a little while after the mass begins to come out through the veins, in order to allow all the capillaries time to fill themselves. When the injection is finished, the organ may be placed to advantage for twenty-four hours in a 2 to 1000 solution of bichromate of potassium in distilled water, and then be removed to alcohol; or it may be put at once in alcohol, and, when hardened, sections made of it. The sections should be pretty thick, and may or may not be stained as is desired. If too thin, they do not show the connections of the vessels.

As an example of the clearness with which the blood-vessels may be traced in a successful preparation, a figure of a section through an injected human lip is given (Figure 76). The shaded portion represents the skin proper, and is penetrated by papillæ sent up from the underlying connective tissue, known in anatomy as the cutis, and carrying the blood-vessels. There is a net-work of small arteries in the cutis, and from this there pass up from three to five fine branches into each papilla, and form by division and inter-communication a wide capillary network. One or several fine capillaries bend

round and form the veinlet which passes down the middle of the papilla, from top to bottom, in a nearly straight line, and some times taking up fine branches on the way until it finally connects with the venous net-work of the cutis.

This arrangement of the vessels is very characteristic ; similar ones occur elsewhere, where there are well-developed papillæ, as, for instance, on the tongue or in the intestine. But each organ presents characteristic peculiarities in the distribution of its bloodvessels, and to an experienced histologist the veins, capillaries, and arteries of the liver and kidney, etc., are as distinctive of each organ as is its general shape and appearance.

As the presence of the valves does not permit us to inject the lymphatics from a large stem in the finer branches, as in the bloodvessels, a different method of forcing in the fluid has to be adopted. A small syringe with a very fine sharp point, such as is known among instrument-makers as a hypodermic syringe, must be used. The point is made to penetrate in the connective tissue, and the colored liquid- the best is a solution of Prussian blue is forced out slowly and gently, and fills at first the cavities of the tissue and then the small lymphatics. These injections are difficult to make and by no means always succeed well. Perhaps the best place to try first is the interdigital web of the hind-foot of a frog, or the outer half, that is, the muscular part of the walls of the small intestine; but the easiest of all to fill are the lymphatics of the dog's testicles. When the injection has been once made in the way indicated, the tissue or organ may be hardened for cutting either in chromic acid or in alcohol.

Such, then, are some of the principal means employed to investigate the microscopical structure of animals. They all have this much in common, that they are endeavors to render certain characters more visible than they are naturally. This we do whether we stain the nucleus, or inject the blood-vessels, or isolate single cells. It may well be added that a good knowledge of optics is necessary to a good histologist.

The worker should also remember that American instruments are usually much less convenient and practical than the German and French microscopes, while the lenses are no better, though enormously more expensive. The writer personally likes Zeiss's instruments very much. As this optician manufactures his objectives upon mathematical principles, he is able to make them all nearly alike; but it must be understood that there are many others whose objectives are also of the best quality. At present

there is no difficulty in getting the best lenses and instruments, providing an American or English microscope of large size and complicated structure is not chosen. It will be found that those only who use a microscope for amusement utterly condemn the simple instruments, while those who make investigations and gather wide experience often assert that the greater the simplicity the better. The European histologists I have met generally use a stand without rack and pinion for coarse adjustment, without movable stage and without movement round a horizontal axis.

As to books, Frey's Manual, of which there has been a translation published in New York, is only pretty good. It came into general use because it was for a long time without rivals. There have lately appeared two little works on this subject, in England, one by Professor Rutherford, the other by Mr. Schaeffer, both of which are considered good. But by far the most important work is Ranvier's Traité Technique d'Histologie now being published in Paris, in numbers, three of which have already appeared. The moderate price of the book, — only twentyfive francs for a volume of a thousand pages, the fullness of detail, and the superb illustrations alone are sufficient to recommend the work. M. Ranvier has written a treatise which will probably always be remembered as one of the most important and valuable manuals ever published, and which ought to be owned by every one who attempts to investigate the elementary structure of animals.

ICH

CONCERNING FOOT-PRINTS.

BY I. C. RUSSELL.

CHNOLOGY (foot-print lore) is the name which has been applied to one of the most attractive and interesting paths of research that geology has pointed out. This branch of palæontology 1 has for its object the study and interpretation of the many fossil foot-prints that have been found in the rocks, which were impressed there by the feet of animals when the material of which those rocks are composed was the shifting sands along some ancient shore. The study of foot-prints has at length been recognized as a distinct and important branch of palæontology, one which has often afforded the only means for judging of the character and structure of the ancient animals that have left no other records of their existence than the impressions of their feet. 1 From palaios, ancient; onta, beings; logos, a discourse.