These days grand rounds in a city hospital may look like ""Father and Son Day at some hippie camp,"" and Dr. Horowitz--one of the longhairs--is no bloodless observer. As a physician he's seen the carelessness and inflexibility rampant in hospitals; he's the first to squawk if an IV is bungled or a patient's dosage is inexplicably tripled. As a writer he offers little that's new, but his droll, cheeky style is a tonic. From his medical school days in Guadalajara, when he literally had to dig up his own cadavers, through an internship and residency in New York, political activism has been in the background. Although his manner is entertaining here (a ""Drunk-sickle"" is an alcoholic who's frozen stiff, ""Mexican heart disease"" is a bullet wound), a commitment to good medicine is clearly his major priority, and as president of his House Staff Association, Dr. Horowitz organized his colleagues in a successful protest against budget cuts--TV cameras and all. No malingering here--strong vital signs and lots of color.