Hi Scott,
I don't know of any full online versions of the Cottingham essay, but doing a search on Google Books will give you a limited preview.
Yes, please post your essay - I'm interested to hear your thoughts.
By the way, since writing the article, it's come to my attention that Descartes almost certainly DID perform vivisection (though possibly not the "nailing cats to trees" variety").
For instance (AT refers to the 12 vol "Oeuvres de Descartes" by Adam and Tannery, by the way):
AT I: he describes how, if you whip a dog whilst playing a violin, it will whimper in time to the music!
AT II 66 (letter to Plempius, 23 March 1638): describes operations on a living eel's heart.
AT XI: 'If you cut off the end of the heart of a living dog and insert your finger through the incision into one of the concavities, you will clearly feel that every time the heart shortens, it presses your finger, and stops pressing it every time it lengthens.'
AT XI 243: describes an operation on a living rabbit.
Letter to Plempius of 15 Feb 1638 (pages 79-85 of Vol III of the Cambridge 'Philosophical Writings' (ed and transl Cottingham et al): "For this is disproved by an utterly decisive experiment, which I was interested to observe several times before, and which I performed today in the course of writing this letter. First, I opened the chest of a live rabbit and removed the ribs to expose the heart and the trunk of the aorta. I then tied the aorta with a thread a good distance from the heart..." (page 81).
Rodis-Lewis, 'Descartes, His Life and Times' (English translation, Cornell, 1995) also assumes that Descartes performed vivisection (see p.85).
(Thanks to various members of the Philos-L philosophy list for pointing out the above.)
So, that sort of clarifies the point that I leave open in the essay, and it also supports the conclusion: the basis on which Descartes ignores animal consciousness is unsound.
Good luck with your essay!