324 VERTEBRATE ZOOLOGY 



greater part of the side of the skull back of the orbits. The optic 

 capsule, which contains the organ of sight, does not ossify, but a 

 membrane bone, the lachrymal, appears at its anterior end, where 

 it forms the anterior wall of the orbit ; the orbit, in which the eye- 

 ball lies, is formed of bones of the brain case and the visceral 

 skeleton. The nasal capsules, with the organ of smell, are bony 

 and cartilaginous and form the anterior end of the skull. 



The bones of the visceral skeleton fall readily into three cate- 

 gories, according to their position : those of the upper jaw and the 

 face, those of the lower jaw, and those of the tongue. 



The Dorsal and Posterior Aspects. The hinder end of the skull 

 is a single bone, the occipital, which has been formed by the fusion 

 of four bones : the ventral basi-occipital ; the two lateral exoccip- 

 itals, which inclose the foramen magnum between themselves and 

 bear the occipital condyles and the long jugal process ; and the 

 dorsal supra-occipital. The last-named bone is much pitted and 

 bears a prominent, posteriorly projecting protuberance to which 

 the muscles are attached which raise the head. At the side of the 

 occipital condyle and between it and the tympanic bulla is a 

 somewhat transverse slit, the jugal foramen, which is the exit of 

 the glossopharyngeal, vagus, and spinal accessory cranial nerves ; 

 in front of this foramen is a large, round hole in the medial wall 

 of the tympanic bulla for the passage of the internal carotid artery. 

 Dorsal to the condyle are the two small hypoglossal foramina, for 

 the exit of the twelfth cranial nerve. 



In the dorsal aspect the small interparietal bone lies in the 

 median area in front of the supra-occipital. In front of it are the 

 two parietals, the two frontals, and the two nasals, these six large 

 membrane bones forming the roof of the skull, the parietals lying 

 just back of the orbits, the frontals between, and the nasals in 

 front of the orbits. The supra-orbital ridge is formed by processes 

 of the frontal bone on each side. Lateral to each parietal, and just 

 back of the orbit, is the large temporal bone, which contains the 

 auditory organ; projecting forward and ventrally from it is its 

 zygomatic process, which forms the hinder part of the zygomatic 

 arch ; and on its ventral side, close to the wall of the cranium, is 

 the glenoid cavity, the articular surface of the lower jaw. 



