THE SKELETAL SYSTEM 11 



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 processels, 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 choanal lies be- 

 tween the anterior extremities of the inner pterygoid proc- 

 esses. 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 premax- 

 illaries), 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 max- 

 illary bears the three molar teeth, which are used for grind- 

 ing 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 



