1 The Birth of Science - MSRI

5.3 Anatomical Terminology and the Screw Press 127
of any mathematical models of physiological processes — have regarded
Euclid’s choice as a “false hypothesis”. 17
One may ask whether ancient optics incorporated other anatomical and
physiological knowledge about the eye. The preface attached to Euclid’s
Optics in the redaction attributed to Theon contains some interesting observations,
such as that, on reading, one’s gaze moves so that the words
read in succession are always centered in the field of vision, and that, analogously,
when scanning for a small object one catches sight of it only when
looking directly at it, at the center of the visual field. Although the text
does not say so explicitly, it is clear that to explain such phenomena in the
framework of Euclid’s optical theory one must suppose that visual rays
are not equidistributed, but more concentrated near the center of the cone
of sight: an assumption compatible with, though not explicitly contained
in, the text of the Optics. 17a The name “weblike” given by Herophilus to
the retina seems to allude to a web’s finer mesh size near the center, so
the idea of a nonuniform distribution of visual rays may have come from
anatomy.
5.3 Anatomical Terminology and the Screw Press
The anatomical terminology introduced by Herophilus can be a precious
source of insights, probably not fully mined yet. For example, he gave page 168
the name pharoid (Pharos-like) to a certain elongated structure, the styloid
apophysis of the temporal bone. Pharos had always been the name of an
Alexandrian islet, but Herophilus had in mind, of course, the new meaning
of the name — the light-house recently built on the island. This shows
a willingness to borrow terms from the new technological reality.
The term torcular Herophili, adopted for the confluence of the cranial
venous sinuses, is particularly interesting in this regard. The Latin word
torcular, from the verb torquere (twist), suggests a screw shape, and this has
always been considered the reason for the name. Because the screw shape
of this structure is characteristic in oxen but not in humans, some have
17 These are Heath’s words (see page 339). Today the term “false” is out of fashion, but Euclid’s
hypothesis continues to appear strange. More recent articles have recognized that Euclidean optics
is in part a theory of visual perception (see [Jones A.] and references therein) but probably we have
not yet exhausted what can be deduced from Hellenistic ophthalmology. See [Medaglia, Russo],
pp. 46–54 for a fuller discussion.
17a The first postulate of the Optics (concerning the structure of visual rays) is certainly corrupt
in the version that has come down to us. In trying to reconstruct it, it may be useful to take into
account the portions of the preface that try to illustrate the postulate’s meaning. The preface is
admittedly late, but some of the notions it contains may predate the corruption of the text. For a
discussion see [Medaglia, Russo].
Revision: 1.9 Date: 2002/09/14 19:12:01