The reference is to the Torso of the Vatican. Haskell and Penny, Taste and the Antique, p.312, cite Aldrovandi in 1556 on Michelangelo 's admiration for the Torso, and quote Wright, Some Observations, Vol. I page 268, published in 1730 as calling the Torso 'the school of Michelangelo'.
Vasari stresses the importance to Michelangelo of the study of the antiques. In the Preface to the Third Part of the Lives he explains that perfection in art had been obtained through seeing 'excavated out of the earth certain antiquities cited by Pliny as among the most famous, such as the Laocoon, the Hercules, the Great Torso of the Vatican, the Venus, the Cleopatra, the Apollo Belvedere, and an endless number of the others':
[They], both with their sweetness and their severity, with their fleshy roundness copied from the greatest beauties of nature, and with certain attitudes which involve no distortion of the whole figure, but only a movement of certain parts, and are revealed with the most perfect grace, brought about the disappearance of a certain dryness, hardness and sharpness of manner which had been left to our art by the excessive study of Piero della Francesca, Lazzaro Vasari, Alesso Baldovinetti, Andrea dal Castagno, Pesello, Ercole Ferrarese, Giovanni Bellini, Cosimo Rosselli, the Abbott of San Clemente (Bartolommeo della Gatta), Domenico del Ghirlandaio, Sandro Botticelli, Andrea Mantegna, Filippo, and Luca Signorelli. Vasari, Le Vite, Testo IV.7).
Reynolds in Discourse Fifteen recommended young artists to study the work of Michelangelo as Michelangelo had studied the works of the ancient sculptors. While Greek sculpture of the Clasical period was unknown in Western Europe, the Torso together with works such as the Venus de' Medici, the Laocoon, and Apollo Belvedere had provided 'the only bulwarks of absolute values' ( Haskell and Penny, Taste and the Antique, p.xiii). Haskell documents the change of taste by which works which had been accepted as masterpieces came to be 'treated with contempt and distaste' as 'for the most part heavily restored Roman copies of Hellenistic originals' (op.cit. page xiii).