Some dogs are deaf from their birth; others are so from some hereditary defect. This remark applies more especially to the white rough-haired terrier.

Sometimes the external canal for the passage of sound is bunged up with morbid growths. Hearing is then more or less impaired in consequence of the obstruction interfering with the transmission of sound. Hearing, of course, returns when this obstruction is removed.

In bad cases of canker, the auditory passage is filled with a mass of morbid growth, which effectually prevents hearing, and which returns, if excised, until the channel becomes obliterated.

The other causes of deafness in dogs are unknown. Only those cases of deafness are curable that depend on a removable cause.