Dorsal notch of foramen magnum due to incomplete ossification of supraoccipital bone in dogs

Abstract

ABSTRACT The shape of the foramen magnum and the variation in the ossification pattern of the supra‐occipital bone were studied in 48 skulls, including 12 from neonatal and 33 from immature and mature beagles of known age. The shape varied markedly, and a dorsal notch, dorsomedial to the nuchal tubercles and present in over half of the beagles, developed as a result of incomplete ossification of the ventromedial part of the supra‐occipital bone. In a pomeranian and a chihuahua a membrane of dure mater and connective tissue, which covered the prominent dorsal notch, prevented prolapse of the brain. The spinal cord passed through a smaller oval opening in the ventral half of the foramen magnum. As there was no apparent impairment of function or anomalies associated with the dorsal notch and since a prominent dorsal notch was common in small breeds with brachycephalic skulls, it was concluded that the dorsal notch is a variation in the normal morphological pattern.