112 ESSENTIALS OF ZOOLOGY 



end of the skull is composed of the exoccipitals lateral to the foramen 

 magnum (opening), supraoccipital dorsal to it, and the basioccipital 

 ventrally. 



The face is composed of the nasals, located just anterior to the 

 frontals, the bones of the jaws, and the zygomatic arches or cheek- 

 bones. The upper jaw is formed by the two anterior premaxillae, 

 and the two longer maxillae. The palatine processes of the maxillae 

 and premaxillae fuse with the palatine bone to form the hard palate 

 or roof of the mouth. The skeleton of the nasal chamber is formed 

 by the vomers and palatines as a floor, ethmoids as walls, and nasals 

 as the roof. The zygomatic arches are composed of the zygomatic 

 processes of the maxillae anteriorly, the malar or jugal bone, and the 

 zygomatic processes of the temporal (squamosal) bones. The lower 

 jaw is composed of the two dentary bones which meet at the anterior 

 midline in the mandibular symphysis. The hyoid bone is U-shaped 

 and lies at the base of the tongue just anterior to the larynx or 

 voice box. 



In addition to the nasal capsule (chamber) mentioned above, there 

 are the optic capsule or orbit and the auditory capsule. The orbit is 

 formed by the frontal bone, the lacrimal, orbitosphenoid, alisphe- 

 noid, and the zygomatic arch. The auditory capsule is enclosed in 

 the tympanic bulla, a part of the temporal bone. 



The rat has only the per^manent set of teeth, and does not have 

 these preceded by the milk or deciduous teeth found in many other 

 mammals. There are sixteen teeth in all, including a pair of in- 

 cisors and three pairs of molars in the upper jaw and the same 

 number in the lower. The incisors, located at the front, are long 

 and chisel-like. The lower pair protrudes farther into the mouth 

 cavity than the upper pair. The upper incisors incline slightly 

 posteriorly. On the jaws just behind the incisors is a toothless 

 space, which is known as the diastema. The molars are broad and 

 cusped for grinding the food gnawed free by the incisors. The 

 number and location of teeth in an animal's mouth are often given 

 in a dental formula which shows the teeth of either the right or left 

 half of the mouth. The formula for the adult rat would be : incisors 

 1/1, molars 3/3 (1/1, 0/0, 0/0, 3/3). This is a marked modifica- 

 tion for the mammals. The dental formula for the cat is: incisors 

 3/3, canines 1/1, premolars 3/2, molars 1/1. For the hog it is: 

 incisors 3/3, canines 1/1, premolars 4/4, molars 3/3 which is the 



