Roman Painting, Milan, Skira Publishing, 1953). Unfortunately, the fresco is in very poor condition, which probably explains why it is more often referred to than reproduced. (See Fishman, G.A., When Your Eyes Have a Wet Nose: the Evolution of the Use of Guide Dogs and Establishing the Seeing Eye. Survey of Ophthalmology 48, 452-458 (July 2003).)
It is not certain the man is blind, and Amedeo Maiuri's narrative does not speculate on blindness, though others have done so. Maiuri, once curator of the Naples Museum, suggests that the man's posture is that of a beggar. Otto Keller, writing almost 50 years before Maiuri, refers to the man as a blind beggar (blinder Bettler) and the dog as little (Hündlein). Otto Keller. Die Antike Tierwelt. Leipzig (1909).
The man is facing two women who, in my opinion but not in Keller's rendition, are watching but not clearly interacting with him. Their reaction to him may have been the dramatic focus of the painting. The scene appears to be set in a market, stalls of which are visible in the background. The man holds a walking stick far forward in his left hand, a posture that may indicate he was using it as much to find obstacles as to provide support. The leash is brownish red, probably a strip of leather, loose, and attached to what appears to be a wide collar. (The only Photoshop enhancement made to the plate posted here was to color the bottom part of the leash, which fades considerably towards the dog's neck.)
The dog, about knee high, looks back at the man, ears pointed like those of a small terrier. In the caption to the drawing he makes of the scene, Keller labels the dog a pariah (Pariahund), though this is perhaps no more than saying that the dog was a street urchin. If the dog is guiding, it probably means that the two are following a route both knew and navigated together by sharing their senses. As sometimes happens with hearing dogs in our time, the dog may have gradually learned to substitute his sense for the one his master was losing.