Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/83

Rh The teeth have been described in connection with the bones of the cranium. There are two long pairs of chisel-like incisor teeth, one pair in the upper and one pair in the lower jaw. The lower pair protrudes farther into the mouth cavity than the upper. The upper incisors incline slightly backward. The lower extend forward from the anterior end of the under jaw, curving upward to their points which lie a little behind the tips of the upper incisors when the jaw is closed. Immediately behind the incisor teeth is the toothless region, the diastema. The three upper and three lower molar teeth for grinding the food torn loose by the incisors, are back of the disastema.

The mouth cavity comprises two parts, the vestibule lying between the teeth on the one hand and the lips and cheeks on the other, and the mouth cavity proper. The vestibule is composed of two completely separated parts—the incisor and the molar. The former lies between the incisor teeth and the lips. On the lower jaw it is well developed, but on the upper is almost completely lacking on account of the cleft in the upper lip. The molar region of the vestibule is a narrow region between the molar teeth and the cheek. The discontinuity of the two parts of the vestibule is due to the absence of teeth in the diastema. The folds of the upper and lower lips and cheek are the only structures on the lateral side of the mouth in the diastema region. The mouth cavity proper is widest in the region of the molar teeth. It decreases in width anteriorly and between the tongue and soft palate extends back as a narrowed passage opening into the pharynx through the isthmus faucium at the posterior end of the soft palate.

The roof of the mouth comprises two regions, the hard palate supported by the palatine processes of the premaxillary, maxillary, and palatine bones, and the soft palate, which consists of a boneless, flexible wall extending back-