When a person thinks of anatomical specimens from the past, he or she may think of disembodied remains floating in glass jars filled with alcohol. The Hunterian Collection at the Royal College of Surgeons in London is full of such specimens—unborn foetuses suspended in time as if still incubating in the womb; a hand, puffy and swollen from chronic lymphedema; a pock-marked face submerged in yellowing liquid.
But not all specimens were preserved in this way. Beginning in the middle of the 18th century, surgeons and anatomists began experimenting with arsenic and mercuric chloride as a means of preserving human remains.
Of course, arsenic had long been used for medicinal purposes before it was ever used for preservative purposes. It was prescribed in small doses for all types of disorders including tuberculosis, rheumatism, and syphilis. During the 15th century, William Wirthering remarked: ‘Poisons in small doses are the best medicines; and the best medicines in too large doses are poisonous.’ 
Soon, however, surgeons and anatomists began recognising the preservative qualities of arsenic when creating dry mount displays from cadaverous remains. Mixtures of arsenic and soap were sometimes used to bathe the insides of a specimen in order to prevent decomposition and insect infestation. This method, invented by the French ornithologist Jean-Baptiste Becoeur (1718 – 1777), was especially popular with taxidermists. In fact, arsenical soap was used in museums around the world until the 1980s when it was finally banned due to its dangerous levels of toxicity.
In 1838, the French chemist, Jean Gannal, introduced a new method for preserving human remains using arsenic as a main ingredient. This was intended to allow anatomists to dissect corpses or prepare anatomical specimens without worrying about putrefaction or decay. He described his method as such:
[A] corpse is injected by the carotid with from five to seven quarts of the acetate of alumina at 20°, and containing in solution about two ounces (fifty grammes) of arsenic acid. Four days after this injection, if it is intended to prepare the large and small vessels, inject by the aorta half a quart of a mixture, equal parts, of the essence of turpentine and essence of varnish; finally, make a single cast of a hot injection of a mixture of suet and of rosin, in equal parts, coloured with cinabar [sic] for arteries, and with a black or blue colour for the veins. Then, the corpse, or the part of the corpse which it is intended to preserve, is prepared and dissected at leisure, according to the wish of the operator. 
By 1846, however, Gannal’s technique was outlawed in France. This was due in part to the fact that anatomists themselves were suffering the effects of arsenic poisoning.
Around the same time, anatomists also began using mercuric chloride as a preservative. The specimens were dipped in the solution, or painted with it. Mercury, as well as arsenic, had also been used for medicinal purposes during the early modern period, when its toxic qualities had not yet been fully recognised.
Today, these types of anatomical specimens pose all sorts of dangers to museum staff who might be handling or interacting with these objects on a regular basis. Both substances can be absorbed through the skin. In cases of arsenic poisoning, severe headaches and confusion first appear, followed by discolouration of skin and fingernails, convulsions, stomach pain, hair loss and eventually death. (Find out here what happens if you ingest arsenic).
Unfortunately, there is no way of knowing whether arsenic or mercuric chloride is present simply by looking at a specimen. Many curators have turned these objects over to laboratories for testing, while others have developed their own methods for determining whether specimens are toxic. Nonetheless, the fact remains that any of the dry mount specimens lurking in museums and private collections are potentially hazardous, as was discovered in this episode of Oddities.
It turns out what you don’t know can in fact hurt you.