BIOLOGY IN STENO'S “CANIS CARCHARIÆ DISSECTUM CAPUT” (1667)

For all the well-founded interest in his pioneering contributions to geology/paleontology, Steno's dissection of the head of a shark, published in Florence 1667 as “Canis Carchariæ dissectum Caput,” has more illustrations and slightly more text on biology than on paleontology. He famously identified the origin of “tongue stones” as fossil shark teeth but the significance to comparative anatomy has received less attention. The biological section was omitted in the first English translation published in 1958 by Garboe. The entire text in Scherz's “Steno – Geological Papers” in 1969 and the editor's commentaries raise the question whether this is just a geological/paleontological treatise.

Among Steno's important observations is the system of mucous canals in the shark's head. He had earlier described this in ray fishes. Later they were to be described in torpedo-fish by his pupil Lorenzini and named after the latter. Steno described a cavity behind the eye of the shark and mentioned, contrary to Aristotle (Hist. Animal. IV/8), that it was possibly an ear - at the location of the hearing organ as identified by much later techniques. Steno marvelled how motions of the huge muscle mass of the shark could be controlled by its tiny brain. He estimated that altogether the cross sectional area of the nerves from the spinal cord are larger by far than the cross sectional area of the spinal cord itself. Thus peripheral nerves could hardly be tubes to carry animal spirits from the brain as held by contemporaries. He added an experiment, much debated by members of the Royal Society of London. Temporary ligature of the abdominal aorta in the awake dog caused temporary hind leg paralysis, the “Steno Experiment,” showing the essential requirement of blood supply for motor control. He described the large number of teeth of the shark and discussed their formation. Further studies of tooth substance and formation, he wrote, could lead to a cure for caries to “diminish the number of the toothless.” Steno's ontological observations were essential in the emerging sciences of paleontology and comparative anatomy and for evolutionary thought.