Sylvester Marsh

“ KNIVES FOR CUTTING SECTIONS. Success in cutting sections depends more upon the excellence of the knife that is used than upon any other one point, and therefore the art of keeping the knife in good order is one which should be acquired by every microscopist, even at the cost of considerable time, study and labor. As a stepping stone to the acquisition of this art, there are certain general principles which must be thoroughly understood before we can hope for success in practice. ”

“ Some are of such a soft and yielding nature that any attempt to cut them is an utter failure, for in place of a perfect section being obtained, nothing remains upon the knife but a mass of diffluent pulp; others, again, are of such density as to resist the action of any cutting instrument. ”

“ The last operation to which the knife is subjected is that of stropping, and as the blade should be stropped after every section, it becomes important that our tools should be good and that we should know how to use them. By having two or three knives we can always avoid grinding and honing the blades ourselves, for it is then easy to hire the work done, and half a dozen knives will last a careful worker a long time with merely the aid of a good strop. ”

“ Preparation of Animal Tissues.—Animal tissues differ from one another so greatly, both in consistence and in chemical composition, as well as in their degree of natural hardness, that no general rules can be given which would be applicable to the preparation of the whole class. Such as are of any considerable degree of hardness, as horn and kindred structures, must be treated much in the same manner as the denser varieties of wood, viz., by more or less prolonged immersion in water—cold, hot, or boiling. ”

“ A most unsuitable arrangement, as a very little actual experience of section-cutting will speedily demonstrate. After many unsuccessful attempts to obtain a really good and reliable section-knife, we determined to have one specially made, which, as it has proved everything that could be desired, merits a brief description. It is of the utmost importance that the blade be made of good and well-tempered steel, not only that it may be capable of receiving an edge of the most exquisite keenness, but also that it may retain it. ”

“ Much may be learned on this point from careful inspection of the edge, and trial on a piece of horn, such as an old razor handle. When drawn over a clean piece of horn once or twice the edge of a soft knife is completely dulled; if well-tempered it should scarcely lose its keenness. Again, when laid flat on the thumb nail and pressed, the edge ought to bend up without breaking or crumbling, and at once, when the pressure is removed, resume its original shape. The extent to which extreme hardness and durability, or toughness, may be combined, is well shown in the famous Toledo sword-blades. ”

“ When the object is very fragile, it is a good plan, after draining off the clove oil, to apply the cover directly upon the section, and then to place a drop of the balsam near to the edge of the cover. This, by capillary attraction, will speedily diffuse itself beneath the cover, flowing over and surrounding the object, without in the slightest degree disturbing its position. If, during the process of mounting, any air-bubbles arise, we may view their development with equanimity, being well assured that as the chloroform evaporates they too will quickly disappear. ”

“ They are careless and in haste, and as a consequence they burn the steel or fail to work it sufficiently, and the result is a useless tool.
The steel may be of the very best quality, however, and well forged, and yet the knife may fail from being badly tempered—too soft or too hard. If too soft, the edge is soon dulled; the knife requires to be frequently honed, and the time wasted in keeping it in order is a serious drawback. If too hard, it is impossible to give it a keen edge, for the metal crumbles away as soon as it is honed or stropped very thin, and the edge becomes ragged and dull. ”

“ Preparation of Vegetable Tissues.—Let us now pass to a brief consideration of the methods usually adopted for preparing the various objects for easy section. In the case of vegetable tissues, not only do we, as a rule, find their texture of too great density to be readily cut in their natural condition, but they also contain much resinous and starchy matter, of which it is highly desirable to get rid. ”

“ When the knife is to be honed, the back is applied and fastened by means of two or three set screws. When laid on the hone, the edge of the knife and the lower surface of the back form the guide and regulate the cutting angle. In this way we can use a broad, thin blade and yet secure great accuracy in honing it. Such a blade has this advantage also, that it is not so liable to be rounded and thus injured by stropping as one in which the relief is obtained by hollowing out the sides. ”

“ Accuracy in a penknife or a razor for shaving is of no consequence; in a section-knife it is a sine qua non.
The surface of the hone must not only be true and smooth, it must also be clean and free from dust and grit, a single particle of which may spoil the work of hours. It should therefore, when not in use, be kept constantly covered. It is always used with oil, and this oil should be of a kind that does not readily dry up. Good sperm is excellent, and so is purified neat’s-foot oil. ”

“ Mucilage, by keeping, is very apt to become sour and mouldy—this may be prevented by adding to each ounce of the water with which it is prepared about half a grain of salicylic acid. We now pour some of this mucilage into a small vessel—an egg cup will answer very well—and into it transfer the piece of ileum from the water. Here we must allow it to remain for a time sufficient to permit of its becoming thoroughly saturated with the gum, for which purpose some hours will be necessary. When this soaking has been accomplished we will prepare the microtome, which we will assume to be Rutherford’s. ”

“ The readiest and most simple plan, if the piece be large enough, is to hold it in the left hand, and, having brought the surface to a perfect 20level by cutting off several rather thick slices, endeavor to cut a thin section by the aid of a very sharp razor, the blade of which must be kept well flooded with spirit. As in the use of Valentin’s knife, so here, great care must be taken steadily to draw the blade across the tissue, every effort being made to avoid pushing the knife, else the section will be torn off, instead of being cut. ”

“ The only protection against this is either to entrust the knife to a man of known carefulness and integrity, or to stand over him while he does it. If the knife be ground under the owner’s eye, no fear need be entertained, because it is easy to insist upon the use of plenty of water.
The grindstone used for section knives should be of fine grain, and it should run true and be very straight across the face, otherwise it will be impossible to grind the knife true, and this, as we have seen, is a necessity. ”

“ Every microscopist that expects to do much at cutting sections must learn to hone his own knife or razor. Of the various hones in use the famous Turkey oilstone is said to be altogether the best. It is, however, very difficult to get it genuine and in sufficiently large pieces. We therefore use the German hone-slate, a softer, yellow stone of wonderful fineness and cutting power. The ordinary whetstones are altogether too coarse, and as for artificial stones and strops, they may do for coarse work and common razors for scraping, but they will not do for knives used for section-cutting. ”

“ The microtome, with its contents, must now be removed to a cool place, when the paraffine will soon become solidified. Whilst this is being accomplished we may make our further preparations. The first thing we require will be a large basin, full of freshly-filtered water, and provided with a cover. A small beaker of methylated spirit, with a dipping rod or pipette, will also be necessary. We must now see that the section-knife is in thorough order, to ensure which it will be advisable to give it a few turns on the strop. An ordinary razor will also be of service. ”

“ But since the steel which we use is limited in its hardness and its power of resisting crushing and 79bending influences, the blades of our knives must have a certain strength or thickness, depending upon the character of the material to be cut. For very hard substances the knife must be stout and the edge ground to a comparatively obtuse angle; for moderately soft substances, such as wood, the angle may be more acute, while for very soft tissues the thinnest blade and the sharpest edge are most suitable. ”

“ Therefore wherever a knife with a flat side is used, no attempt should ever be made to grind this flat side on the hone. Such an attempt will almost always result in an edge so irregular that it will be almost impossible to cut a good section with it. But if the under side of the knife be left truly flat, we will have the very best tool that can be had for use with the microtome. ”

“ Unless removed it will very speedily ruin a fine knife, therefore look out for it and wipe off the hone carefully if you have any suspicion that particles of steel have broken off the blade and got on to it.
In honing, as in everything else, however, nothing but practice will impart skill, and he who intends to become an expert should practice on a few old razors, grinding, honing 91and stropping them himself, until he has acquired the art of giving them an edge far keener than most of the razors ordinarily used for shaving. ”

“ A very simple imbedding agent, and one of the greatest practical value, is a strong solution of gum arabic, which, upon being dehydrated either by ordinary drying or the action of alcohol, soon acquires such a degree of hardness as to permit it (with the imbedded tissue) to be cut with ease. ”

“ It is a very easy matter to obtain from the butchers the foot of a sheep which has just been killed. The joint is to be opened, and the bones dissociated, when they will be seen to have their extremities coated with a white glistening membrane—this is the articular cartilage. 61Exceedingly thin slices must be at once cut from it, and as only small sections are required, a sharp razor may be used for the purpose, the blade being either dry or simply wetted with distilled water. ”

“ The points which specially demand attention in a knife for cutting sections are these: 1. Quality of the steel used; 2. Temper; 3. Form of the blade.
Of the quality of the material of which a knife is made, nothing can be determined except by actual trial. The old tests of staining with acids, examining with the microscope, etc., are worthless, or at least too crude and uncertain to be of any practical value. Color changes with the degree of polish 85that is produced, and, in short, there is no reliable guide. The purchaser must depend entirely upon the reputation of the manufacturer. ”

“ At the outset we may say that unless the student intends to devote himself solely to the production of sections of wood, etc., he ought not to procure 22one of those forms of microtome known as wood section-cutters, in which the object to be cut is held in position in the tube by means of a binding screw which pierces its side. [4] Although these machines are all that can be desired for cutting hard bodies, they are not so suitable for soft ones. ”

“ The denser the tissue, the thinner should the section be; whilst certain substances of loose and spongy texture do not require the sections to be particularly thin—it may be said, however, in a general way that sections, and especially animal ones, cannot be cut too thin so long as they remain perfect and entire. If Professor Rutherford’s microtome (as made by Gardner) be employed, the head of the screw will be found to be graduated into divisions of slightly unequal value ”

“ Liver.—Small pieces of liver may be very successfully hardened by immersion in alcohol, beginning with weak spirit and ending with absolute alcohol. Cut and mount as usual.
35. Lung must be prepared in chromic acid (§ 5) . For the cutting of sections the freezing microtome (§ 18) is of especial value, and should, therefore, be used. If, however, the student be not provided with this instrument, he must proceed as follows. A small piece of lung, previously deprived of all spirit, is to be immersed until thoroughly saturated in solution of gum (§ 18) . ”

“ The best vessels in which to stain sections are small jars of white porcelain, capable of holding about two fluid ounces, and furnished with lids—they are much preferable to beakers or watch glasses, for owing to the white background which they afford it is very easy to watch how the staining is proceeding. The carmine solution which we have just described is both too strong and of too great density to be used in its pure state. ”

“ It is very easy to discover if this condition be fulfilled, for if on carefully laying the flat of the blade upon a piece of level glass, every portion of both back and edge are found to be in close contact with it, the knife may in this respect be considered perfect. Every student who aspires to be a successful section-cutter should provide 26himself with a good Turkey oilstone, and learn to use it. He should also possess a razor strop, as it will be in constant requisition. ”

“ Any intelligent worker in brass would make an instrument of this kind at a very small cost, and although perhaps it might lack the finish of an instrument bought at the optician’s, it would, if accurately made, do its work as well as the most complicated and expensive. If, however, the student resolves to purchase a microtome, there are a variety of forms in the market from which he may choose. ”

“ After carefully wiping the knife from all shreds of paraffine, the microtome screw must again be partially revolved, more spirit applied to the tissue, and another section being cut, it must be transferred to the water as before, and so on, until a sufficient number of sections have been obtained. As to how thin the sections should be cut, no general directions can be given; each case must be regulated by its own conditions. ”