Troy Oviatt of the Fairfield, Calif., Major Crimes Unit said the month-old boy was in stable condition at Oakland Children's Hospital, where doctors determined he also suffered a skull fracture and brain hemorrhage, the Los Angeles Times reported.
Oviatt said police were called to child's home in Fairfield early Thursday by the baby's hysterical 17-year-old mother, who reported that the baby was bleeding from the nose.
Doctors said about a third of the infant's nose had been severed.
Joshua Cooper, 18, of Fairfield, was arrested on suspicion of child cruelty and aggravated mayhem, the Times said.
Cooper was taken to the Solano County Jail.
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