Page:Laboratory Manual of the Anatomy of the Rat (Hunt 1924).djvu/25

Rh At the posterior end of the skull is the foramen magnum, through which the brain connects with the spinal cord. On each side of this foramen is the occipital condyle. Two pairs of bony plates, the pterygoid processes, arise at the base of each alisphenoid bone. The outer plates are nearly horizontal in position. They extend from the posterior end of the maxillary bone caudad and laterad to the lateral surface of the tympanic bulla. The two inner pterygoid processes are vertical to the base of the cranium. Each extends posteriorly to the opening of the bony canal through which the Eustachian tube passes from the cavity of the middle ear (within the tympanic bulla) to the pharynx. The single posterior naris, or choana lies between the anterior extremities of the inner pterygoid processes. It leads forward into the nasal cavity, which opens at the anterior end of the head through the two anterior nares.

The roof of the mouth in front of the choana consists of three pairs of bones (palatines, maxillaries, and premaxillaries), which taken together constitute the hard palate. At the extreme posterior end of the hard palate are the two small palatine bones, which form the ventral boundary of the choana. The maxillary bones are considerably larger than the palatines and are located laterally and anteriorly to the latter. The lateral, or alveolar, portion of the maxillary bears the three molar teeth, which are used for grinding food. The maxillaries articulate in front with the pre-maxillaries. These bear the two large curved incisor teeth. Canine and premolar teeth are absent in the rat.

Several pairs of foramina appear on the ventral surface. The twelfth (hypoglossal) cranial nerve emerges through the hypoglossal canal at the base of the occipital condyle. The posterior lacerated foramen lies immediately in front of the hypoglossal canal, at the boundary between the