THE INTERIOR OF THE SKULL 



113 



a section will divide the parietal bones slightly posterior to the parietal eminences, 

 and a line drawn transversely across the section at the mid-point will give the 

 greatest transverse measurement of the cranial cavity. A skull divided in this 

 way facilitates the examination of the parts about the choanae (posterior nares) . 



The horizontal section (figs. 139, 140) of the skull should be made through a 

 line extending from the ophryon to the occipital point, passing laterally a few 

 millimetres above the pterion on each side. It is of great advantage to study the 

 various parts on the floor of the cranial cavity in a second skull in which the dura 

 mater and its various processes have not been removed. 



The floor [basis cranii interna] of the cranial cavity presents three irregular 

 depressions termed the anterior, middle, and posterior fossae (figs. 139 and 140). 



The Anterior Cranial Fossa. — The floor of this fossa is on a higher level 

 than the rest of the cranial floor. It is formed by the horizontal plate of the 

 frontal bone, the cribriform plate of the ethmoid, and the lesser wings of the 



Fig. 138. — The Skull in Sagittal Section. 



Bregma 



Ophryon 



Gonion 



Basion 



sphenoid, which meet and exclude the body of the sphenoid from the anterior 

 fossa. The free margins of the lesser wings and the anterior margin of the optic 

 groove mark the limits of this fossa posteriorly. The central portion is depressed 

 on each side of the crista galli, presents the numerous apertures of the cribriform 

 plate, and takes part in the formation of the roof of the nasal fossae; laterally, 

 the floor of the anterior cranial fossa is convex; it forms the roof of the orbits, 

 and is marked by irregular furrows. It supports the frontal lobes of the cerebrum. 

 The sutures traversing the floor of the fossa are the fronto-ethmoidal, forming 

 three sides of a rectangle, that portion of the transverse facial suture which tra- 

 verses the roof of the orbit, and the ethmo-sphenoidal suture, the centre of which 

 corresponds to the gonion. The other points of interest in the fossa are: — 



A groove for the superior sagittal sinus. 



The foramen caecum which frequently transmits a small vein to the nasal cavity. 



The crista galli. 



The ethmoidal fissure for the anterior ethmoida^l branch of the fifth nerve. 



The cranial orifice of the anterior ethmoidal canal, transmitting the anterior ethmoidal 

 branch of the fifth nerve, and a meningeal branch of the anterior ethmoidal artery. 



The cranial orifice of the posterior ethmoidal canal, transmitting a meningeal branch of the 

 posterior ethmoidal artery. 



The ethmoidal spine of the sphenoid. 



Furrows for meningeal vessels. 

 8 



