156 THE VERTEBRATE SKELETON. 



The foramina lie however very close together and are some- 

 times confluent. The cranial walls for a considerable distance 

 in front of the occipitals are unossified, but the anterior end 

 of the cranial cavity is encircled by another cartilage bone, the 

 sphenethmoid (figs. 24 and 25, 1) or girdle bone. This 

 partly corresponds to the orbitosphenoids of the Newt's skull. 

 Anteriorly it is pierced by a pair of small foramina through 

 which the ophthalmic branches of the trigeminal nerve pass out. 



The anterior part of the cranial cavity is divided into two 

 halves by a vertical plate, the mesethmoid. Some little 

 distance behind the sphenethmoid the ventro-lateral walls of 

 the cartilaginous cranium are pierced by a pair of rather pro- 

 minent holes, the optic foramina (figs. 24 and 25, B, n), and 

 at a similar distance further back, occupying a kind of notch 

 in the pro-otic are the large trigeminal foramina, through 

 which the fifth and seventh nerves leave the cranium. Between 

 the trigeminal and optic foramina are the very small foramina 

 for the sixth nerves (fig. 25, B, vi). 



The membrane bones of the cranium proper include the 

 fronto-parietals and the parasphenoid. 



The fronto-parietals (figs. 24 and 26, A, 2) form a pair of 

 long flat bones closely applied to one another in the middle line, 

 the line of junction being the sagittal suture. They cover 

 over the fontanelles and overlap the sphenethmoid in front. 



The parasphenoid (figs. 24 and 26, B, 7) is a bone shaped 

 like a dagger with a very short handle. It lies on the ventral 

 surface of the cranium, the blade being directed forwards and 

 underlying the sphenethmoid; its lateral processes underlie 

 the auditory capsules. i 



(2) THE SENSE CAPSULES. 



The sense capsules are cartilaginous or bony structures 

 which surround the olfactory and auditory organs and are 

 closely united to the cranium. 



The auditory capsules are fused with the sides of the 



