32 MANUAL OF ELEMENTARY ZOOLOGY 



cartilage bone known as the sphenethmoid. This has the 

 form of a dice box divided across the narrowest part by a 

 transverse partition which closes the cranial cavity in front. 

 A longitudinal partition divides the front half of the box 

 into two. The roof of the cartilaginous cranium is pierced 

 by three large holes or fontanelles, but these are not seen 

 in an intact skull, since the whole roof is covered, from 

 the exoccipitals to the sphenethmoid, by two long bones, 

 the frontoparietals, placed side by side. The floor is com- 

 plete, and under it lies a large dagger-shaped bone, the 

 parasphenoid, placed with the blade of the dagger for- 

 ward and the crosspiece of its hilt under the auditory 

 capsules. 



The wall of the cranium is pierced by certain openings 

 or foramina, for the passage of the nerves which arise from 

 the brain. These " cranial " nerves are ten in number on 

 each side. The first nerve of each side passes through a 

 foramen in the transverse partition of the sphenethmoid on 

 its way from the organ of smell in the nasal capsule. The 

 second nerve, which serves the eye, enters the skull through 

 a conspicuous opening on each side in the middle of the 

 sphenoidal region. The third and fourth nerves have each 

 a minute foramen in the side of the same region. The 

 fifth and seventh nerves pass through a large common 

 opening on the under side of the skull, situated in a notch 

 in the prootic bone mentioned below. The foramen 

 for the sixth nerve is a small opening between those for 

 the second and for the fifth and seventh. The eighth 

 nerve enters from the inner part of the ear by an open- 

 ing in the wall between the cranium and the auditory 

 capsule. A foramen for the ninth and tenth nerves is 

 situated in the exoccipital bone, at the side of the occipital 

 condyle. 



The nasal capsules are a pair of irregular, mainly 

 cartilaginous enclosures continuous with the front end of 

 the cranium. Only their hinder part is ossified, and this 

 forms that part of the sphenethmoid which lies in front of 

 its transverse partition. The wall between the two capsules 

 is known as the mesethmoid. Through these capsules run 

 the passages from the nostrils to the mouth, and each 

 of them has therefore an opening above and below. Each 



