We carried out a computerised tomographic (CT) examination to elucidate
the modifications in the head
related to orbital enlargement in the Baikal seal. Transverse CT images
showed that (1) the external frontal
contours and the frontal sinuses are compressed medially and ventrally
by the orbital enlargement; (2) the
caudal part of the nasal cavity and nasopharynx are compacted ventrally;
and (3) the cranial cavity is
displaced caudally. The neurocranium is obviously separated from the facial
part in the transverse plane at
the caudal region of the orbit. The disposition of the mandible, zygomatic
arch, temporal bone, and the
masseter, temporal, digastric and pterygoid muscles is changed by the enlarged
orbit in the 3-dimensional
reorganisation of the head in this species. It is suggested that adaptation
for the Lake Baikal environment
primarily resulted in orbital enlargement, and that the altered orbital
design may subsequently have
influenced the form and function of the masticatory and respiratory system.