Andrew Eastbourne has come through, and a .doc file of this text (De officina medici) arrived today and can be downloaded from here: Galen_-_Preface. I have also uploaded it to the Fathers site here. I’m placing this in the public domain — do whatever you like with it (except stick your own copyright notice on it!)
It is most interesting as a guide to the transmission of texts in ancient times, so I will do my best to post it here.
He entitled a medical [work], “Pertaining to the Surgery” (κατ’ ἰητρεῖον). But it would have been better for it to be entitled, “On the Things Pertaining to the Surgery” (περὶ τῶν κατ’ ἰητρεῖον), as some give the title for the [works] of Diocles, Philotimus, and Mantius. For while these men wrote on the same subject, in each book, in the greatest number [of copies] the title lacks the preposition (περί) and the article (τῶν)—they are entitled, simply, “Pertaining to the Surgery”—in a few [copies], however, [it is given] with the preposition and the article: “On the Things Pertaining to the Surgery.” But whereas these men’s books give quite copious theoretical instruction, Hippocrates’ [book], after the catalogue of the things that are the components of surgery overall, gives a full explanation of bandaging, since the man considered it proper to practice this first. And indeed, the practice of this can be pursued most especially with pieces of wood sculpted into human form, or if [this is] not [possible], on the bodies of children at least.
This much the book itself required me to say, before my interpretations of individual points; now, however, I will go through what is not required by the book, but by those who, in copying  them, readily received the writings of the ancients in whatever [form] they themselves wished. For some eagerly attempted to find 300-year-old copies of even very old books, preserving some in papyrus scrolls, others on sheets of papyrus, others on parchment, like the [texts] that are with us in Pergamum.
Therefore, I decided to examine all these things in the [commentaries of the] earliest interpreters, so that on the basis of the majority and the most trustworthy I might discover the authentic writings. And the result turned out to surpass my expectations. For I discovered that they nearly all agreed with each other—the treatises and the commentaries of the interpreters—such that I was struck with bewilderment at the audacity of those who have recently written commentaries or have made their own edition of all the books of Hippocrates, among whom are Dioscorides and his associates, and Artemidorus, called Capito, and his associates, who made many innovations in the ancient writings.
It seemed to me that the account of the commentaries would be [too] long, if I mentioned all the writings, and so I imagined that it was better to write [about] the older ones only, adding to them some few others—those that show but little alteration—and of these, primarily those which have been acknowledged by the earlier commentators on the book. There are four of them: two, who wrote commentaries on all the books of Hippocrates—Zeuxis and Heraclides; and then Bacchius and Asclepiades, [whose comments], not on all [the books of Hippocrates, are] hard to understand.
And now, enough of these matters. By way of recovering the pleasure of a clearer exordium, I will speak briefly, as though I had not said anything already. Hippocrates’ book, entitled “Pertaining to the Surgery,” contains at the outset a preamble to the whole art [of medicine], as I shall demonstrate a little later, and for this reason some have reasonably considered it proper to read it first of all, promising lessons very similar to what some later gave in the works they entitled “Introductions.” And next in sequence after the common preamble, he teaches (regarding what can be effected in the surgery) the most useful things for those who are beginning to learn the medical art. It will become plain to you that [all] this is the case as you apply your mind carefully to the explanations of the expressions themselves.
From: Claudii Galeni Opera Omnia, tom. XVIII pars II, ed. D. Carolus Gottlob Kühn, Lipsiae (1830), p. 629-632. Title: ΤΟ ΙΠΠΟΚΡΑΤΟΥΣ ΚΑΤ̕ΙΗΤΡΕΙΟΝ ΒΙΒΛΙΟΝ ΚΑΙ ΓΑΛΗΝΟΥ ΕΙΣ ΑΥΤΟ ΥΠΟΜΝΗΜΑ Α. The title of the Latin translation is: Hippocratis De Medici Officina liber et Galeni in eum Commentarius I; Galeni praefatio. [Note by R.P.]
 “Surgery” here appears to refer to the physical set-up for a doctor’s operations, not the practice of surgery to which the English term most frequently refers.
 The Greek term, μεταγράφοντες, carries the implication that they changed them in the process of copying.
 Here Birt, Das antike Buchwesen, p. 503, suggests emending the odd ἢ (“or” [?]) to οἳ, yielding the following meaning for the sentence: “…but by the copyists, who readily took…”
 In the Greek, it is the copying rather than the composition that is explicitly described as “300-years old,” since the participle γεγραμμένα—lit., “having been written”—is in the accusative case, whereas the books are in the genitive.
 Kuhn’s text (τὰδὲἐνδιαφόροιςφιλύραις, ὥσπερτὰπαρ’ ἡμῖνἐνΠεργάμῳ: “others on various / excellent [sheets of paper made from] the under-bark of the lime tree, like the texts that are with us in Pergamum”) is problematic. Although this under-bark is attested as being used for writing (Herodian 1.17.1; Cassius Dio 72.8.4), it has no connection with Pergamum. Birt, Das antike Buchwesen, p. 503, cites Cobet’s emendation (ἐνδιφθέραις) with approval—I have adopted it here; Birt also mentions Marquardt’s suggestion (ἐνδιφθερίναιςφιλύραις: “on [sheets of] parchment ‘bark’”).
 The phrasing here—”Dioscorides and his associates” (Gk. οἱπερὶΔιοσκορίδην)—is frequently used in Greek as a circumlocution for the simple “Dioscorides.”
 Gk. δυσλόγιστα; this can mean, literally, “hard to calculate” or “bad at calculating” and hence, either obscurity or shoddy commentating is the point.
UPDATE: Andrew Eastbourne writes to remind me that “duties” of a doctor would be “officiis”, and to say that “officina” is workshop/laboratory.