Head 
anterior nasal septum, as far caudally as the pala- 
tine fissure (Figs. 2-10 to 2-15, 2-29, 2-30). They 
are involved in the sense of smell, since they are 
supplied by tw^igs of the olfactory nerve (Nickel, et. 
al, 1960). Each measures 5 mm in length, while 
the width ranges between 0.4 to 1.0 mm (Jensen, 
1977). Rostrally it has a laterally compressed, 
eliptical cross-section, which caudally assumes a 
half-moon shape, convex medially. The organ is 
invested by a bony covering of the vomer, and com- 
municates with the incisive duct (Fig. 2-14), ending 
blindly slightly caudal to that structure. 
The vomeronasal organs are lined by two types of epithelium. The 
ventral margins of the glands are covered by epithelium similar to the 
olfactory region of the nose, including olfactory sensory cells inter- 
spersed throughout the supporting columar epithelium, which is non- 
ciliated pseudostratified and free of goblet cells. The dorsal edges are 
continuous with the respiratory epithelium of the nasal cavity, which 
is pseudostratified, ciliated, columar and interspersed with goblet cells 
(Fig. 2-40). 
T~he initial part of the nasal cavity, the nasal vestibule (vestibulum 
nasi) approximately 4 mm in length, is lined with stratified squamous 
epithelial mucosa. The nasal cavity proper, the nasopharynx and the 
nasal septum are lined with a respiratory mucosa consisting of cili- 
cated pseudostratified columnar epithelium with goblet cells (Figs. 
2-40, 2-41, 2-42). The ethmoturbinais are invested with olfactory 
epithelium (Fig. 2-38). 
2.6 PARANASAL CAVITY 
The European hamster has only one true para- 
nasal cavity (sinus paranasalis), the maxillary sinus 
(sinus maxillaris) (Figs. 2-16 to 2-18, 2-31, 2-32). 
It is surrounded by the outer and inner surfaces 
of the maxillary bone and extends 4.5 mm from the 
level of the union of the ventral nasal meatuses to 
an imaginary line drawn perpendicular from the 
orbits to the lower jaw. Two portions can be dis- 
tinguished, a smaller rostrodorsal portion 2.5 mm 
high and 1 mm wide, and a larger caudoventral 
portion, 5 mm high and 1 mm wide (Jensen, 1977). 
The maxillary sinus communicates with the nasal 
cavity via the nasomaxillary opening (apertura 
nasomaxillaris) into the middle nasal meatus. 
The paranasal sinus is lined with pseudostratified ciliated columnar 
epithelium containing goblet cells (Figs. 2-43, 2-44). Directly beneath 
this epithelium are situated glandular bundles (100 //m in diameter) 
composed of serous and mucous parts. In the oral region these glands 
demonstrate a more mucous character, whereas caudally they contain 
more serous parts. 
2.7 ORAL CAVITY 
The oral cavity (cavum oris) extends from the 
lips (labia oris) to the oropharynx. Its bony struc- 
ture consists dorsally of paired incisive and maxil- 
lary (Figs. 2-2, 2-8) and single palatine bones, and 
ventrally of the paired dentary bones constituting 
the mandibles (mandibulae) (Fig. 2-45). The oral 
cavity is bordered rostrally by the lips and laterally 
by the cheeks (buccae). The roof of the oral cavity 
consists of the hard palate (palatum durum) and the 
soft palate (palatum molle) (Figs. 2-3, 2-46). The 
oral cavity is floored by the tongue and by the 
reflections of mucous membrane extending from the 
tongue to the gum (gingiva) on the medial surface 
of the mandible (Fig. 2-46). 
The oral vestibule (vestibulum oris), between the 
teeth and lips, is separate from the oral cavity 
proper (cavum oris proprium); however, the sepa- 
ration is incomplete due to the long diastema in the 
tooth row. Associated with the presence of cheek 
pouches is a longitudinal separation of the vestibule 
into buccal and labial parts (vestibulum buccale, 
vestibulum labiale). The separation, better devel- 
oped in the mandibular than the maxillary vesti- 
bule, results from a fold of oral mucosa (tunica 
mucosa oris). Median labial frenula (frenulum labii 
maxillaris, frenulum labii mandibularis) separate 
maxillary and mandibular labial vestibules into 
left and right halves and serve to bind the lips 
tightly to the gums. 
2.8 LIPS 
The entrance of the oral cavity (rima oris) is en- 
closed by the upper lip (labium maxillare) and the 
lower lip (labium mandibulare) . The relatively 
short lips cannot be completely closed, so the inci- 
sors always remain visible. The upper and lower 
lips join at the labial commissure (angulus oris), 
20 to 25 mm caudal to the mandibular incisors. The 
commissure is actually a compound structure, with 
maxillary and mandibular portions, separated by a 
mucosal fold. The resulting valve-like structure 
controls entrance into the cheek pouches, as in the 
Syrian golden hamster (Schwarze and Michel, 
1959-60). The lips are not confined to the rima 
oris, but project prominent flaps of hair-covered 
skin, the buccal pads (pulvini buccales) into the 
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