28 AN INTRODUCTION TO ZOOLOGY 



In life this space is covered by a tough skin, the atlanto-occipital 

 membrane. The exoccipital not only forms the back of the cranium, 

 but also the back and part of the floor of the auditory capsule, which 

 forms a prominent structure fixed to each side of the hinder end of 

 the cranium. The front wall of the capsule is formed by another 

 cartilage bone, the pro-otic, at whose lower inner corner is situated 

 a foramen, through which the fifth, sixth and seventh nerves given 

 off from the brain leave the skull. The lateral wall of the auditory 

 capsule, its roof and floor, are formed of cartilage more or less 

 covered by superposed membrane bones. In the posterior part of 

 the lateral wall is a small opening, the fenestra ovalis, leading to the 

 sensory part of the ear. This is closed by a small block of cartilage, 

 the stapes, to which is attached a short thin bony rod, the columella, 

 and this in its turn is connected with the tympanum. 



From the auditory capsule the cranium is continued forward as a 

 cartilaginous tube to the front end of the orbit. Half-way along 

 its side it is perforated by a large circular foramen, through which 

 the second nerve from the brain passes to the eye. The dorsal 

 surface is also incomplete, and is perforated at the front end by a large 

 hole, the anterior fontanelle, and at the hinder end by two smaller 

 holes, the posterior fontanelles. The fontanelles are completely 

 hidden by the paired overlying membrane bones, the fronto-parietals, 

 which spread out slightly at the posterior end and help to cover the 

 inner part of the roof of the auditory capsule. The cartilaginous 

 floor of the cranium is similarly covered by a membrane bone, the 

 unpaired parasphenoid, a T-shaped bone, whose arms underly the 

 whole of the bases of the auditory capsules. 



The anterior end of the cranium is formed by the girdle bone, 

 or sphenethmoid. This is a cartilage bone completely encircling 

 the cranium, and it has a deep hollow at each end. The partition 

 between the two cavities is perforated for the passage of the nerves 

 of smell. It is a bone that is peculiar to the Amphibia. The cavity 

 at the front end is divided into two, a right and a left, by means of 

 the cartilaginous internasal septum, which passes forwards and 

 completely separates the two olfactory cavities from one another. 



The front end of the cranium bears the two olfactory capsules, 

 two incomplete cartilaginous structures. Over the dorsal part of the 

 capsule is situated a triangular membrane bone, the nasal, and just 

 in front of this is a very small bone, the septomaxillary. A triangular 

 membrane bone, the vomer, lies beneath the capsule and bears at 

 its hinder inner corner a small group of pointed teeth, the vomerine 

 teeth, while its hinder outer corner is deeply notched to form the 

 boundary of the internal nostril. 



The outline of the lower part of the skull is arch-shaped, and 



