CT examination of the head of the Baikal seal (Phoca sibirica)

We carried out a computerised tomographic (CT) examination to elucidate the modifications in the head related to orbital enlargement in the Baikal sea l. Transverse CT images showed that (1) the external frontal contours and t he frontal sinuses are compressed medially and ventrally by the orbital enl argement; (2) the caudal part of the nasal cavity and nasopharynx are compa cted ventrally; and (3) the cranial cavity is displaced caudally. The neuro cranium is obviously separated from the facial part in the transverse plane at the caudal region of the orbit. The disposition of the mandible, zygoma tic arch, temporal bone, and the masseter, temporal, digastric and pterygoi d muscles is changed by the enlarged orbit in the 3-dimensional reorganisat ion of the head in this species. It is suggested that adaptation for the La ke Baikal environment primarily resulted in orbital enlargement, and that t he altered orbital design may subsequently have influenced the form and fun ction of the masticatory and respiratory system.