Pig. 14. Base of Skull seen from above. 



Cochlea and Semicircular Canals, on the right side, exposed. 



The base of the Skull is more complicated and irregular than the Vertex. 

 Three Fossae are recognised, the anterior bounded behind by the sharp border 

 of the lesser wing of the sphenoid, in this lies the Frontal I^obe, in the middle 

 line are found the Crista Galli and the Cribriform Plate of the Ethmoid through 

 which the Olfactory Nerves pass. 



The Middle Fossa extends from the Sella Turcica on either side being 

 limited by the upper border of the Petrous portion of the Temporal Bone. This 

 Fossa contains the Temporal Lobe and many Foramina through which the vessels 

 and nerves pass. 



Optic Foramen for Optic Nerve and Ophthalmic Artery. 



Sphenoidal Fissure for Superior Ophthalmic Vein, VI th, I Vth and Ilird 

 and Ophthalmic Division of Vth Nerve. 



Foramen Rotundum for 2nd Division of Vth Nerve. 



Foramen Ovale for 3rd Division of the Vth Nerve. 



Foramen Spinosum for Middle Meningeal Arterj'. 



The Middle Lacerated Foramen is closed by f ibro-cartilage , a 

 remnant of the primary cartilaginous skull. The Posterior Fossa contains the 

 Cerebellum and Bulb ; its boundaries are the Petrous Bones and the Lateral Sinus. 

 The Facial and Auditory Nerves leave the skull b}' passing through the Internal 

 Auditory Meatus ; the Internal Jugular Vein, Glossopharyngeal, Vagus and Spinal 

 Accessor}' Nerves by passing through the Jugular Foramen and the Hypoglossal 

 Nerve through the Anterior Cond3-lar Foramen. 



When the base of the skull is held up to the light, the bones are seen 

 to vary much in thickness. The stoutest portions are: great wing of Sphenoid, 

 Basisphenoid, Basiocciput and Middle ^/^rd of Petrous Bone. The thinnest are : 

 Cribriform Plate, Sella Turcica (because the Sphenoidal Sinuses are beneath), 

 lateral parts of middle fossa, Tegmen T3'mpani, region over Temporo-maxillary 

 articulation and floor of the Posterior Fossa. Violence, broadl}- speaking, can 

 only affect the base indirectly in as much as it is ever3'Where protected by the 

 parts covering it. At a few places only is it exposed to trauma: at the Cribriform 

 plate if foreign bodies are pushed into the Nasal Cavities, at the roof of the orbit 

 if foreign bodies are pushed upwards into the orbital cavity. 



The excavated right petrous bone shews the Internal Ear, Cochlea, Internal 

 Auditory Meatus, and 2 Semicircular Canals (the Anterior, Vertical; and the Internal, 

 Horizontal). The 3rd or Posterior Canal is not shewn. 



