386 THE VERTEBRATE SKELETON. 



foramen magnum (fig. 75, 2) through which the brain and 

 spinal cord communicate. Forming the lower margin of the 

 foramen magnum is a large flat unpaired bone, the basi- 

 occipital (fig. 75, 5). Above this on each side are the ex- 

 OCCipitals, whose sides are drawn out into a pair of down- 

 wardly-directed paroccipital processes, which are applied 

 to the tympanic bullae 1 . The inner side of each exoccipital is 

 converted into the large rounded occipital condyle (fig. 72, 

 13) by which the skull articulates with the atlas vertebra. The 

 dorsal boundary of the foramen magnum is formed by a large 

 unpaired flat bone, the supra-occipital (figs. 72 and 75, 1), 

 which is continuous with a small bone, the interparietal, pro- 

 longed forwards between the parietal bones of the next segment. 

 In old animals the interparietal forms the hind part of a 

 prominent ridge running along the mid-dorsal surface of the 

 skull and called the sagittal crest, while the junction line of 

 the occipital and parietal segments forms a prominent occi- 

 pital crest. 



The plane in which the bones of the occipital segment lie 

 is called the occipital plane ; the angle that it makes with the 

 basicranial axis varies much in different mammals. 



The parietal segment consists of both cartilage and 

 membrane bones. It is formed of five bones, which are in 

 contact with those of the occipital segment on the dorsal and 

 ventral surfaces, while laterally they are separated by the inter- 

 position of the auditory bones, and to some extent of the squa- 

 mosal. The basisphenoid (fig. 75, 6), an unpaired bone 

 forming the ventral member of this segment, is the direct 

 continuation of the basi-occipital. It tapers anteriorly, but is 

 rather deep vertically, its upper or dorsal surface bearing a 

 depression, the sella turcica, which lodges the pituitary body 

 of the brain. From the sides of the basisphenoid arise the 

 alisphenoids (fig. 75, 11) a pair of bones of irregular shape 

 generally described as wing-like ; each gives off from its lower 

 1 See p. 392. 



