34 ELEMENTS OF MAMMALIAN ANATOMY 



theory is no longer held, being contrary to well-established 

 facts of comparative anatomy and embryology. Neverthe- 

 less a study of the make-up of these rings will be a convenient 

 way of introducing the names and locations of the bones of 

 the skull. 



The most posterior or caudal of these rings may be called 

 the occipital segment. It consists of four bones completely 

 fused together to form a single bone. These are the hasiocci- 

 pilal, the two rxoccipituls, and the supriwccipital. Each of 

 these take part in forming the border of the foramen magnum, 

 although the exoccipitals have the greatest share. 



Immediately anterior to the occipital ring is the parietal 

 segment. It is formed of five bones: the basis phenoid, two 

 alisphenoids, and two parictals. The occipital and parietal 

 segments are separated ventrolaterally by a hiatus into which 

 is wedged the auditory capsule. 



The most anterior of the rings which form the wall of the 

 brain case proper is the frontal segment. It, too, consists of 

 five bones: the presphenoid, two orbitosphcnoids, and two fron- 

 lals. Like the occipital segment it is also separated ventro- 

 laterally from the parietal by an open space which in this case 

 is the orbital fissure, an exit for several nerves from the brain. 



In addition to the three segments mentioned above, there 

 arc three sense capsules. 



The olfactory capsule forms the most anterior of the cranial 

 segments and may be called the olfactory segment. It consists 

 of two tubular cavities open at each end. Each of these cavi- 

 ties is j^artially divided into an upper olfactory chamber, closed 

 behind by the cribriform plate; and a lower nasal chamber, 

 terminating in the choancc. 



The inner wall of each cavity is formed mainly by the 

 mesethmoid and vomer. The outer wall is formed mainly by 

 the pterygoid, the palatine, the maxilla, the premaxilla, the 

 lachrymal, and a portion of the frontal. The roof is formed by 

 the nasal process or the frontal and the nasal. The horizontal 



