CHAPTER XX 



THE SKULL 



The skull consists of the protective case round the brain 

 (neurocranium) and of the skeletal supports of the jaws 

 (splanchnocranium). It is formed in all chordates from 

 Petromyzon upwards (whence the name Craniate) and is always 

 cartilaginous at first. In Cyclostomes and Selachians the 

 skull remains cartilaginous throughout life, but in other forms 

 this cartilaginous chondrocranium becomes more or less 

 thoroughly replaced by cartilage-bone, and membrane-bones 

 are added to it. The chondrocrania of the various vertebrates 

 may be compared with one another on the one hand, and on 

 the other, the bony skulls may similarly be compared. 



Cartilaginous Skull. — The typical structure of the chondro- 

 cranium may now be considered. The floor of the neuro- 

 cranium is formed of paired trabecular in front (enclosing the 

 hypophysial fenestra between them) and of paired parachordals 

 (on each side of the notochord) behind. The auditory capsules 

 are firmly anchored on to the parachordals on each side. 

 Behind the auditory capsules the paired occipital arches rise 

 up from the parachordals, and become attached to the hind 

 part of the auditory capsule. In so doing they enclose a 

 fenestra (metotica) through which the glossopharyngeal and 

 vagus nerves and the internal jugular vein pass. In front of 

 the auditory capsule paired pillars rise up from the para- 

 chordals and join on to the orbital cartilages. The latter 

 form the sides of the brain-case in front of the auditory capsules, 

 and the pillars just mentioned are the pilae antoticae. The pila 

 antotica joins the front part of the auditory capsule of its own 

 side, and in so doing encloses the trigeminal, facial, and 



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