Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/37

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The palatine bone, membranous in origin, consists chiefly of three elements—the horizontal part, and the orbital and sphenoidal processes. The horizonal part constitutes the posterior region of the hard palate, and therefore is the ventral boundary of the choana. It meets its fellow of the opposite palatine bone at the median suture along the dorsal side of the mouth cavity. Laterally it articulates with the alveolar process of the maxillary bone. The orbital process, which joins the horizontal part nearly at right angles, is exposed to the orbito-temporal fossa. It constitutes the greater part of the nasopharyngeal duct's lateral wall. The dorsal edge articulates with the presphenoid bone. The sphenoid process extends backward to the pterygoid process, and joins dorsally an anterior extension of the alisphenoid bone.

The mandible comprises two lateral elements united in front by connective tissue at the mandibular symphysis. Each element contains five parts: the body, the ramus, and three processes attached to the latter posteriorly. The body of each half of the mandible is prolonged anteriorly as a strong, curved process bearing one large incisor tooth. This tooth projects anterodorsally a considerable distance beyond the bone. Behind the incisor is the diastema, a toothless region corresponding to the diastema of the upper jaw. A row of three molar teeth is posterior to the diastema. The two rows of molars are approximately parallel to each other, but each row forms an acute angle with the long axis of the bone in which it is inserted. There are transverse ridges on the grinding surfaces of the molars. The ramus is the laterally flattened region immediately