82 



THE SKELETON 



measurement of the cranial cavity. A skull tlivided in this way facilitates the 

 examination of the parts about the posterior nares. 



The horizontal section of the skull should be made through a line extending 

 from tlie ophryon to tlic 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 (»f tiie cranial cavity in a second skull having the dura mater and its 

 various j)rocesses in situ. 



The floor of the cranial cavity presents three irregular depressions termed the 

 anterior, middle, and jiosterior fossae. 



The Anterior Fo.ssa. — 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 j^late of the ethmoid, and the lesser wings of the sphenoid, which meet 

 each other and exclude the pre-sphenoid from the anterior fossa. The free margins 

 of the lesser wings and the optic groove mark the limits of this fossa ])Osteriorly. 

 The central portion of the fossa is depressed on each side of the crista galli, the 



Fig. 90.— The Skull in Sagittal Section. 



BREGMA 



^-^OPHRYON 



OH EXTERNAL 



OCCIPITAL PRO 



TUBERANCE 



depressions forming a part of the roofs of the nasal sinuses; laterally, the floor of 

 this fossa is convex where it corresponds to 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 ])ortion of the transverse facial suture which traverses the roof of 

 the orbit, and the ethmoido-sphenoidal suture, the centre of which corresponds to 

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



A groovi! for the su]ierior longitudinal sinus. 



The foramen caecum which transmits a small vein. 



Tlie crista galli. 



Tlie ethmoidal fissure tor the nasal branch of the fifth nerve. 



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

 branch of tlie fiftli nerve, and a inriiiuMcal brancli of the anterior ethmoidal artery. 



Ethmoidal foramina for the olf,\(tory filaments. 



Cranial orifice of thr posterior ethmoidal canal, transmitting a meningeal 

 branch of the i)Osterior ctlmioidal artery. 



