1 88 VERTEBRATE SKELETON 



inclined backwards or may be nearly horizontal. The centre of the 

 basioccipital is the vertebral part of the basal plate and the bone 

 extends forwards into the interotic region. It bears part of the occip- 

 ital condyle in a few species and occasionally (some whales) is 

 excluded from the foramen magnum. The exoccipitals bear the 

 somewhat oval condyles, their major axes diverging dorsally. In 

 front of each condyle is usually a foramen (hypoglossal canal) for 

 the twelfth nerve, but in Monotremes the nerve leaves by the jugular 

 foramen. Usually each exoccipital bears a wing-like paramastoid 

 fparoccipital) process for muscular attachment on its lateral side 



Fig. 200. — Diagram of bones of mammalian skull (altered from Flower). Cartilage 

 bones dotted; membrane bones lined; 2-12, nerve exits. 



(fig. 205), this being large in rodents and many Ungulates, etc., 

 reduced or lacking in Primates, whales and Sirenians. 



The supraoccipital is the largest of the occipitaha. In lower 

 mammals it ascends obKquely forwards, in Primates backwards and 

 upwards in accordance with the increase in the cerebellum. Usually 

 it is increased in extent in the adult by the union with it of the 

 interparietal. 



The sphenoidalia ossify in the basal plate, trabeculae and ali- and 

 orbitosphenoid cartilages. Both basi- and presphenoid have paired 

 centres of ossification (presphenoid possibly having two pairs). 

 The basisphenoid is usually the larger and bears the sella turcica on 

 its dorsal surface, this being bounded behind by the dorsum sellae, in 

 front by the sellar tubercle which separates the two optic foramina 



