INTERIOR OF THE SKULL. 61 



The upper part of the cranial cavity forms an unbroken arch ; the lower 

 part is divisible into three parts or fossae having different levels. 



The anterior fossa, formed by the orbital plates of the frontal bone, the 

 small wing of the sphenoid, and the cribriform plate of the ethmoid, sup- 

 ports the anterior lobes of the brain. It is deepest opposite the ethmoid 

 bone, and convex over the orbits. It is pierced by the foramina of the 

 cribriform plate, and by the foramen coecum in front of the crista galli of 

 the ethmoid bone. 



The middle fossa presents a mesial and two lateral parts. The mesial 

 part is small, being formed by the olivary process and sella turcica of the 

 sphenoid bone, and limited behind by the dorsum sellse. The lateral part 

 on each side, formed by the great wing of the sphenoid, the squamous part 

 of the temporal, and the anterior surface of the petrous part, lodges the 

 middle lobe of the brain. The foramina of the middle fossa are the fora- 

 men opticum, the sphenoidal fissure, foramen rotund um, foramen ovale, 

 foramen spinosum, foramen lacerum medium, and hiatus Fallopii. 



The posterior fossa t deeper and larger than the others, extends back to the 

 occipital protuberance, and lodges the cerebellum and medulla oblongata. 

 The occipital bone and the petrous and mastoid portions of the temporal 

 bone take part in its formation. In the posterior surface of the pars pe- 

 trosa, which limits this fossa anteriorly on each side, is the internal audi- 

 tory meatus, below it is the foramen lacerum posterius, below and internal 

 to that is the anterior condyloid foramen, and in the middle line is the 

 foramen magnum. 



Grooves for Bloodvessels. The groove of the middle meningeal artery 

 commences at the foramen spinosum of the sphenoid bone, and ramifies 

 principally on the squamous portion of the temporal bone and on the 

 parietal. The groove of the internal carotid artery lies on the side of the 

 body of the sphenoid bone, and terminates on the internal margin of the 

 anterior clinoid process. The groove of the superior longitudinal sinus, 

 commencing at the foramen coecum in front of the crista galli of the ethmoid 

 bone, enlarges as it passes backwards in the middle line of the roof of the 

 skull, and terminates at the internal occipital protuberance. At that point 

 commence the grooves of the lateral sinuses, which pass outwards on the 

 occipital bone, cross the posterior inferior angles of the parietal bones, 

 descend on the mastoid portions of the temporal bones, run inwards again on 

 the occipital, and turn forwards to terminate at the jugular foramen. The 

 groove of the inferior petrosal sinus lies between the petrous portion of the 

 temporal bone and the basilar process ; that of the superior petrosal sinus 

 extends along the superior angular edge of the petrous portion. 



THE NASAL CAVITIES AND COMMUNICATING SINUSES. The nasal cavities 

 are placed one at each side of the middle line, being separated by a vertical 

 septum. They open in front and behind by the anterior and posterior 

 nares already described, and communicate by foramina with the sinuses of 

 the frontal, ethmoid, sphenoid, and superior maxillary bones. Their vertical 

 extent, as well as that from before backwards, is considerable, but the 

 transverse width of each is very limited, especially in the upper part. 



The internal wall, or septum narium, is formed principally by the central 

 plate of the ethmoid bone and the vomer ; but the frontal spine, the rostrum 

 of the sphenoid bone, and the crests of the maxillary and palate bones 

 likewise enter into its composition. It presents a great angular deficiency 

 in front, which in the recent state is filled up by the septal cartilage. In 



