Fig. 49.—Weighted Cork for Dissecting Trough
The object to be dissected is placed in the trough, secured in position by means of a few ordinary pins, and then completely covered with water.
We need hardly impress upon the reader the great importance of thoroughly examining all external characters—all those structures that are visible without actual dissection—before attempting to remove anything; and we have already insisted on the importance of carefully examining all creatures while alive before anything else is done. The value of this latter stipulation can hardly be overestimated, for in many instances it is almost impossible to detect the use of an organ unless it has been observed in action; and the enthusiastic student will go even further than this, for he will make it an invariable rule to sketch everything he sees, and to make full notes on all his observations.
When pins are used to fix the object under examination—and it is generally essential that the object be fixed—their heads should be turned outwards; for then the object will not slip from its position, nor will the pins tend to get in the way of the work.
Some objects are of such a nature that they are not easily secured by means of pins, and yet require to be fixed in some way or other. Thus, one may desire to examine the structure and appendages of a prawn or small crab, or to investigate the nature of a chiton. In such instances as these it is a good plan to make a cake of paraffin wax of suitable size by pouring the melted substance into a mould, then secure the object in proper position in the wax while still fluid, and pin the latter to the cork of the dissecting trough.
It is often necessary to trace the courses of internal passages that open on the surface of the body, or of tubes that are revealed during the progress of dissection, and this may be done by means of a little instrument called a seeker. It is simply a blunted needle, bent into a large angle, and mounted in a handle; or, it may consist of nothing but a moderately long and stiff bristle, rendered blunt at one end by tipping it with melted sealing wax. This is not always sufficient, however, for it frequently happens that certain tubes and passages in animal forms are disposed in such a complicated manner that it is impossible to send even the most flexible seeker through them. For instance, suppose one desires to trace the course of the digestive tube of some large bivalve mollusc with its many reflections, the seeker is useless except that it will penetrate to the first sharp bend. The arrangement of such a tube must be traced by dissecting along its course, but this may be aided considerably by first filling it with some coloured substance to enable its direction to be more easily followed. In fact, the injection of some brightly coloured fluid, forced through the tube by means of a fine-nozzled glass syringe will often enable the course of such a tube to be seen without any dissection at all, the colour of the fluid used being detected through the semi-transparent tissues surrounding it. A mixture of Berlin blue and water, or a mixture of plaster of Paris and water coloured with carmine is well adapted to this purpose; and if the latter is employed it may be allowed to set, and thus produce a permanent cast from the tube that is being dissected. Perhaps it should be mentioned that if either of the injection mixtures be used for this purpose it must be previously strained through muslin, and that, in the case of the plaster, the mixing and straining should occupy as little time as possible, or it may begin to set before the injection has been completed.
A very considerable insight into the structure of animals may be frequently obtained by cutting sections through the body with all its organs in situ, but, generally speaking, they are too soft to allow of this without danger of the displacement of those very parts, the relations of which we desire to determine. To avoid this the body should be previously hardened by a somewhat prolonged soaking in methylated spirit, or in a solution of chromic acid prepared as before directed. Then, with the aid of a good razor, very interesting sections may be prepared with the greatest of ease, and the true relations of the various organs throughout the body may be exactly determined by cutting a succession of slices, not necessarily very thin, from end to end, or, transversely, from side to side.