THE ENDOSKELETON: SKULL AND VISCERAL SKELETON 109 



lateral regions, which constitute the two halves of the upper jaw. The upper 

 jaw forms the margins of the skull and consists in part of tooth-bearing bones. 

 The teeth are arranged in two rows. About the middle of the margin is a project- 

 ing process; on examining the ventral surface of this it will be seen to bear a 

 fossa for articulation with the lower jaw. At the posterior end of the skull is 

 an opening, the foramen magnum, through which the brain is continuous with 

 the spinal cord. On either side ventral to the foramen magnum is a projection, 

 the occipital condyle, bearing a smooth face for articulation with the atlas. On 

 each side of the foramen magnum and extending anteriorly is an expanded 

 region with partially cartilaginous walls. This is the otic capsule fused on each 

 side to the occipital region of the skull. At the anterior end of each otic capsule 

 is a depression for the passage of nerves. Anterior to this depression is another, 

 the orbit, in which the optic nerve runs to the eyeball. Anterior to the orbit is 

 a slitlike cavity, the base of the cavity of the olfactory capsules. 

 2. Bones of the skull proper. 



a) Membrane bones of the roof of the skull: The roof of the skull is formed by 

 two pairs of membrane bones, in the median region a posterior pair of parietals 

 and an anterior pair of frontals. The parietals extend from the dorsal rim of 

 the foramen magnum to a little beyond the middle of the roof where they termi- 

 nate in a median point. The frontals lie on each side of and partially cover the 

 anterior end of the parietals and extend forward nearly to the tip of the skull. 



b) Membrane bones of the floor of the skull: Examine the ventral surface. It 

 is composed almost entirely of the very large parasphenoid or parabasal bone 

 extending from the occipital condyles forward nearly to the tip of the skull. 

 It is shaped somewhat like a short-necked bottle, the neck of the bottle lying 

 between a pair of bones, the vomers, which complete the sides of the anterior 

 part of the ventral surface. Each vomer bears teeth on its margin and extends 

 to the dorsal side, forming the floor of the nasal capsule. On the ventral side 

 between the anterior parts of the vomers and in front of the termination of the 

 parasphenoid is a cartilage, the ethmoid plate, which is the anterior part of the 

 chondrocranium. 



c) Cartilage bones of the chondrocranium and otic capsules: The chondro- 

 cranium is ossified only at its posterior end, all the rest of it remaining in the 

 cartilage stage. The ossification consists of a pair of exoccipital bones which 

 bear the occipital condyles. The exoccipitals, besides bearing the occipital 

 condyles, form the lateral walls of the foramen magnum; the dorsal rim of the 

 foramen magnum is formed by a strip of cartilage, the synotic tectum, which is 

 partially covered by the posterior ends of the parietals. 



The otic capsules are partially ossified each containing two cartilage bones, 

 the opisthotic and the prootic. The opisthotic is lateral to the exoccipital and 

 is a cone-shaped bone composing the projecting angle of the skull noticeable on 

 each side of the occipital region. The dorsal portion of the opisthotic articulates 



