PLATES I, II, III.

EXTRANEOUS MATTERS.

I BELIEVE that the greatest difficulty with which the student has to compete when he commences to examine objects in the microscope, arises from the presence of foreign substances, which have become accidentally associated with the specimen under examination. The microscopical character of many of these extraneous matters is very striking, and unless the observer is familiar with their appearance he is very likely to mistake them for structures which he is looking for. Indeed, practical observation will alone enable any one to describe and interpret with confidence what he has seen under the microscope. He must not be led into the error of giving a name to what he sees, because it appears to him to resemble in appearance a structure of which he has merely read a description, or of which he has only seen a bad drawing.

I think it will be admitted that I am not overstating the importance of this matter, when it is stated that portions of feather have been gravely described and figured as lymphatic vessels, fragments of vegetable hairs as portions of nerve fibres, and numerous other blunders, not less culpable or ridiculous, have been made and promulgated. Of all substances likely to be examined by the medical practitioner, urinary deposits are liable to contain numerous structures which are of accidental

PLATE II.

EXTRANEOUS MATTERS COMMONLY FOUND AMONGST URINARY DEPOSITS.

Fig. 6. Potato starch. Distinguished by its ovate form, and by the hilum, or point around which a number of concentric lines are arranged, being situated near one extremity.

Fig. 7. Wheat starch. Distinguished by its circular form. The hilum is seldom visible, but its situation is central.

Fig. 8. Rice starch. Distinguished by the very minute size and irregular form of the corpuscles.

Fig. 9. Testa of wheat, which forms the external covering of the grain.

Fig. 10. Bread crumbs.

Fig. 11. Cells of potato, in which the starch is contained. A few of the cells are filled with starch granules.