Tuatara: Volume 7, Issue 3, June 1959
Selecting Dissecting Instruments for General Zoology
Selecting Dissecting Instruments for General Zoology
The purchase of a set of dissecting instruments becomes essential when the student approaches his first year of university work in zoology. This is the time when a little care and thought will lead to the selection of a limited number of instruments which with few additions will serve adequately for many years whether the student continues in zoology or goes on to veterinary science, home science, dentistry, medicine, agriculture, etc.
A minimum of good quality instruments of suitable shapes and sizes, kept in good condition and correctly used, are a prime essential to the aret of dissection, and will prove an economy of time, energy and money. The work of the first year in zoology will include the dissection of animals ranging in size and nature from the small soft-bodied earthworm to the large hard-skinned shark, and other animals such as the frog, and a mammal which might be a rabbit, a white rat or a guinea-pig. A limited number of simple instruments will prove adequate for all this work and for the work of other courses in later years.
It is extremely difficult to describe all the characteristics which enable an experienced person to distinguish instruments of good quality from those which are poor in design, made of cheap materials and with faulty workmanship. Scalpels come in all grades from cheap, case-hardened, all-metal scalpels which are brittle and most difficult to sharpen, to forged scalpels with finely-tempered, high-quality steel blades which sharpen readily to a perfect cutting edge. Avoid stainless steel scalpels which do not take or keep an edge, and those which fail to ring sharply when the scalpel is held lightly by the end of the handle and the tip of the blade is clicked with the thumbnail or tapped on a hard surface such as glass. Avoid scissors which are unduly heavy, thick in the blades or handles, and which fail to cut cleanly at the tip. If you have the opportunity, check a variety of scalpels, forceps, scissors, etc., and try to develop a sense of balance in these instruments. A good all-metal scalpel balances at the middle of the total length; well-designed scissors balance about two-thirds of the length from the tip. Well-made forceps balance at about the mid-point in their length. The double thickness at the hinge in well-designed forceps is thinner than the mid-portion of the arm, and the portion between the hinge and the modelled arm will be the narrowest and the page 100 thinnest part of the arm (see detail, Fig. E), even thinner than the portion used to form the hinge since it is by thinning and narrowing this region that the designer creates a pair of forceps with a really light spring action.
Most dissection instruments are plated. Good plating is smooth, without pits, cracks or ripples, uniformly and brightly reflective. Chrome plating has a bluish tinge, an oily appearance and a long life in ordinary usage. Nickel plating is silvery and with proper care has a reasonably long life but tends to peel when it breaks. Chrome-plated instruments are preferable but usually somewhat more costly than nickel-plated instruments of similar quality. Even when the plating is worn, a good instrument is fully usable provided it is not allowed to rust.
An adequate dissecting set will include one scalpel; fine and heavy scissors; fine and coarse forceps; two dissecting needles; a probe; a six-inch ruler; dividers; pencil; rubber. Hooks and chains and the blowpipe are described because they are still supplied in many kits which persuades students that these items are necessary, but this is not correct. Hand-lens, pipettes, pins, slides and cover-slips, corded hooks, and other items used from time to time throughout the year are not included here as part of the dissecting kit since these and a sharpening stone are generally supplied to the student in the laboratory as required.
This is a first requirement. Dissection instruments are sharp tools. Edges and points blunt and damage if the instruments are kept loose in a box. Expensive sets are supplied with wooden boxes fitted with racks, but soft cases with pockets or loops for each instrument are less costly and fully satisfactory. Equally suitable is the instrument roll made of heavy linen with pockets or loops and a turn-in flap, much to the pattern of a carpenter's wood-bit roll. A roll can be easily made to the pattern illustrated in Fig. H.
The most costly case or the simplest home-made roll is useless unless instruments are kept in it and in their proper places when not required.
LEGEND TO FIGURES
A, scalpel and cross-section through blade; B, heavy dissection scissors; C, light dissection scissors; D, fine dissection forceps; E, heavy dissection forceps, and side view of detail of hinge-end; F, detail of interlocking ridges on tips of forceps; G, german-silver wire probe; H, plan layout for instrument roll;. Note: Scale for Figs. A to G, 2 in. Plate prepared by Mr. J. A. F. Garrick.
The sharp edge of the blade should be used only to do the work which the other parts of the scalpel will not perform. This keeps the edge of the blade sharp and saves sharpening. A scalpel with a straight edge is designed for cutting down through thick layers or across solid structures and has little value in general dissection because only a small part of the edge can be used. If the sharp edge is convex there is a greater length of usable edge and as one portion dulls, a change in the angle of attack will allow another portion of the edge to come into use. Keep the edge sharp. Heavy cutting pressure with a dull scalpel can lead to accidents and damage to the dissection or to yourself.
From the above account of the use of a scalpel it can be seen that a single scalpel suitable for a wide range of general work should have a convex blade about l½ in. long by at least ½ in. at its greatest depth. The handle should be at least 4½ in. long so that the scalpel can be reversed and the end of the handle used in dissection without danger to the fingers from the cutting edge.
A good scalpel has a blade which is an acute-angled triangle in section. A deeply hollow-ground blade sharpens easily for light work but will be found too weak for heavy tasks.
Sharpen the scalpel on a fine grade india stone wet with water. Lay the blade flat. Rub the blade lengthwise. Then reverse and rub the other side an equal amount. Take out nicks by using a coarser grade of stone.
Avoid scalpels with replaceable blades. These were not originally designed for general dissection, are too fragile for much heavy work, cannot be sharpened, and usually provide only a short cutting edge.
If anything, these are more generally useful cutting instruments than is the scalpel; but it requires some experience before scissors are freely and properly employed in dissection.
Two types of scissors are required. One is a smaller pair with sharp- page 103 Dissecting Instruments for Zoology pointed blades from 1 in. to 1 ½ in. in length, and the whole instrument of at least 4 in. total length; the other, a larger pair of scissors about 6 in. in total length with strong blades 2 in. long and preferably one blade tapering and sharp-pointed, the other blade being almost parallel-edged and rounded at the end. Obviously, the smaller are used for finer dissection, the larger for heavier work. Scissors with curved blades are of little general use and are employed for trimming away loose strands or edges of sheets of tissue.
It is most important in scissors that they cut to the very tips of the blades when used. Good scissors cut cleanly a piece of paper even less than 1 mm. from the tip, and scissors should be tested this way before purchase.
Dissection of the frog provides a good opportunity for the diversified use of scissors. A scalpel is used in this dissection only when removing the bones forming the roof of the skull. The skin on the frog is divided, as required, by picking up a small fold of the skin in fine forceps, cutting it across just sufficiently to allow entry of the tip of the small scissors and then cutting lengthwise using the greater length of the blade of the scissors. The same scissors are used to cut through the septa which attach the skin to the muscular body-wall. The same technique is used in opening the body cavity; but the heavy scissors are used with the blunt blade within the animal to cut through the pectoral girdle. The blunt end of the blade tends to force aside soft structures with little damage. A sharp pointed blade used in the same way may pierce a structure so that it will be cut or otherwise damaged. Forceps and scissors are used again in dissecting the leg to expose the sciatic nerve passing to the gastrocnemius muscle. Scissors are used to open the pericardium to expose the heart, and to open the heart to examine the chambers and valves.
These are used as an extension of the fingers. They permit the hand to be kept out of the immediate field of the dissection and are used for the grasping of small or large items often for considerable periods of time such as when a structure is kept under tension to assist its dissection, as in exposing a nerve or a blood-vessel passing through connective tissues. Typically a pair of forceps consists of two parallel metal blades or arms fastened permanently at one end as a spring-hinge and ending at the other with a tip of various forms, straight or curved, blunt or sharp.
For general dissection, two pairs of forceps are required, a pointed pair for finer work, a blunt pair for heavy work. In both cases select only forceps which close under light pressure. Forceps having a strong spring hinge rapidly tire the hand. The pressure to close good forceps is so little that it can hardly be appreciated, yet on release the tips spring wide apart, 3/4 in. or more. Good forceps of all sizes have a guide-pin, a small tapering pin mounted on the inner face of one arm and passing through an opening on the opposite arm when the forceps are closed. This small pin should be sturdy since it lines up the tips when closed. It makes the tips meet accurately even under heavy pressure. The inner face of each tip should page 104 be ridged cross-wise for at least 1/4 in. to ½ in. The ridges should fit into one another when the tips are closed. This adds greatly to the grip. (Toothed forceps are unsuitable in the dissection of smaller animals with delicate tissues.) The blades or arms should not be too narrow or sharp-edged, or otherwise uncomfortable to hold for any length of time.
Pointed forceps for fine work are held between the thumb and forefinger, or grasped in the hand and so can be used at many angles. For this reason straight forceps are preferred. Forceps with curved tips are more restricted in use and less satisfactory also, because unless a very expensive, well-built pair is obtained, the points tend to separate or twist apart under pressure.
Blunt forceps should be really sturdy, but still only lightly sprung so that when held between thumb and forefinger or grasped in the hand they close easily and open readily to at least 1 in. between the tips. They should be straight, plain enough to be comfortable to hold but with some modelling to fit the hand so that they will not tend to slip if wet and greasy. Some well-built heavy forceps have a low flat-topped stop on the inner face of one arm in addition to the guide-pin. This stop prevents the tips from separating under heavy pressure, but if the stop is placed near the middle of the arm or nearer the tip than the hinge, it is useless.
Because forceps primarily enable the hand to be kept clear from the field of dissection and forceps are used in a variety of positions, both fine and blunt forceps should be at least five to six inches in length.
Avoid forceps which have a strong spring; in which the tips close under light pressure but open at the tips when full pressure is applied; which lack guide-pins; in which the ridging of the tips does not interlock.
Dissection needles are steel needles mounted in the ends of rounded wooden handles. These instruments are more neglected and abused than any other. Simple as these instruments are, they are worthy of respect. They take the place of fine forceps and a scalpel in the minutest dissection such as under the hand-lens. Experienced workers use only a pair of needles to carry out dissections by hand even under the low power of the ordinary microscope.
The needles are used, one in either hand, held between thumb and forefinger and supported on the index finger. One needle is pressed down on to a specimen to hold it in place. The sharp tip of the other needle is pressed lightly on to and drawn along the specimen as though it were a scalpel, and properly used it will cut into the specimen. This is the best way to remove the wings and legs of a small insect such as a fly or a mosquito, to free internal organs in a weta to expose the nervous system, to dissect the head of a preserved small tadpole which is a simpler and clearer dissection than the head of a frog, etc.
Good dissecting needles are less than a millimetre in diameter, sold for ordinary sewing as size 5/9. A needle 1 ½ in. long is of a convenient length. Needles of this size are sufficiently rigid and yet flexible enough to be page 105 useful when pressed on to a specimen to hold it down. They are flexible enough for sharpening. This is done by holding the handle against the tips of the fingers with the thumb so that the handle can be rotated by rolling it with the thumb. The needle is pressed on to a fine stone so that about ½ in. of the needle is in contact with the stone. By a combination of rubbing the needle lengthwise and rotating it on the stone, a long, tapering, extremely sharp point can be formed. Needles usually supplied for dissection are too rigid for proper sharpening. A well-sharpened needle can hardly be felt if it is pushed into the finger.
Probe or Seeker
In its best form, this is nothing but a piece of german-silver or silver wire about 6 in. long, a good millimetre in diameter, straight or curved as shown (Fig. G), and bluntly rounded at each end. It should be semi-soft, soft enough that it can be pushed gently inside a blood-vessel or duct, and tend to follow the path of the structure. The probe then acts as a guide for scissors which can be used to open the length of the vessel. This is almost a forgotten technique in ordinary dissection and yet it is the simplest way to locate and follow structures such as the anterior cardinal vein in the dogfish, or the ureters in a fatty rat, etc.
The hard-drawn blunt steel wire mounted in a round wooden handle is of little practical use. The instrument in this form has discouraged the use of the probe in dissection; but a good probe is a most useful aid in many dissections.
This was a common instrument in former years when dissections were carried out on fresh material. The blowpipe consisted of a metal tube tapering to about 2 mm. in diameter. This narrow end could be passed into the bladder, into the intestine or other hollow organs and ducts, which could then be inflated to display form and connections. Since most larger dissections are now performed on preserved material, the blowpipe has dropped from use.
Hooks and Chains
These are seen in the large complete dissecting sets such as contain a variety of scalpels, forceps, etc. There is usually a pair in a set. Each consists of three pieces of light chain about 6 in. long joined to a ring and the free ends armed each with a strong sharp hook. They were used in the dissection of large animals, animals of sufficient size that two of the hooks could be separated and driven into the flesh of the animal as anchors for the third hook which could be used to hold an organ or an edge of a flap of body wall, etc., clear from the dissecting field. They are not sufficiently adaptable for use on small animals since they cannot be conveniently page 106 shortened as required and it is for this reason they are not suitable for anchoring a small animal to the dissecting board. Purchase of hooks and chains is not worth while until a student begins dissection of large animals when they may be found really usable.
Ruler, Dividers, Pencil and Rubber
These can hardly be termed dissecting instruments but they have a proper place in any dissecting kit since there is no better way to study a dissection than to prepare a well-proportioned and detailed drawing of the work that has been done. The simplest way to do this is by a mechanical transfer of the proportions of major structures from the dissection to paper. Decide a size for the drawing, a reduction or magnification such as will allow all detail to be conveniently represented. Then take the length and breadth of the various major structures item by item, using the dividers, measuring each on the ruler, reducing or enlarging each according to the scale, and mark out the dimensions in their relative positions on the drawing before attempting the outlines. Outlines should be lightly dotted in first and corrected until a good representation has been obtained. This may appear tedious, but it is the most accurate and the fastest way to obtain a good drawing.
A 6 in. rule is all that is needed; celluloid and plastic are preferable to wood, but wood is adequate. The dividers should have legs at least 6 in. long, the hinge should be firm and capable of being tightened if necessary. A good pencil, kept sharp, is needed. HB or softer grades are unsatisfactory because the drawings rub when assembled into a book or folder, and drawings made with soft pencils smear even to the extent of becoming illegible. Use a 2F or 2H pencil.
Before you buy your rubber, check to see that it does not leave a mark on the paper. A good grade of soft red rubber is adequate.
|(1)||Instruments loose on the bench are dangerous. Place instruments in the case when not in use. This keeps instruments in the best condition.|
|(2)||Do not abuse your instruments. The scalpel was not made for sharpening pencils. Fine forceps and needles are used for dissection, not for playing darts.|
|(3)||Sharpen your instruments before they become dull. This means better dissection, easier sharpening, and a longer life for the instrument.|
|(4)||Do not let instruments get rusty. Rust rapidly destroys sharp points and cutting edges. Clean and dry each time before putting instruments away. This is extremely important after using them in or near salt water.|
|(5)||Do not use fine grade instruments for coarse dissection.|