THE UPPEE SUEFACE OF THE BASE OF THE SKULL. 179 



slightly to the lateral side of their extremities. The student will note that there 

 is no hole in the jugular process of the occipital bone corresponding to the arterial 

 foramen in the transverse process of the atlas through which the vertebral artery 

 passes. The course of this vessel over the upper surface of the posterior arch 

 behind the superior articular processes of the atlas will be seen to coincide with 

 the posterior condylic fossse and the margins of the foramen magnum immediately 

 medial thereto, where a slight grooving of the edge often indicates the course of 

 the artery. In front the anterior tubercle of the atlas falls in line with the 

 pharyngeal tubercle on the under surface of the basi-occipital, and the student must 

 not overlook the fact that the anterior surface of the cervical column does not 

 coincide with the anterior margin of the foramen magnum, but lies nearly half an 

 inch in front of that, in a frontal plane passing immediately in front of the 

 external acoustic meatuses. Behind, the upper surface of the posterior arch of the 

 atlas overlaps the posterior margin of the foramen magnum, and it is by the 

 apposition of these two surfaces that extension is checked at the occipito-atlantal 

 articulation. 



THE SKULL IN SECTION. 



By the removal of the skull-cap the cerebral aspect of the cranial cavity is exposed. 

 The deep surface of the cranial vault is grooved in the median plane for the superior 

 sagittal sinus, on either side of which are seen numerous depressions for the lodgment 

 of arachnoideal granulations. On holding the bone up to the light, the floor of these 

 little hollows is oftentimes seen to be very thin. A short distance in front of the 

 lambda, and on either side of the sagittal suture, are the cerebral openings of the 

 parietal foramina. The inner tables of the frontal and parietal bones are grooved 

 for the meningeal arteries. The principal branch of the middle meningeal runs more 

 or less parallel to and at a variable distance behind the line of the coronal suture. 

 Along the bottom of these grooves small foramina may be seen for the passage of 

 nutrient arteries to the bone, and the floor of the sagittal sinus is likewise pierced 

 by small apertures for the transmission of veins. 



Basis Cranii Interna. 



Cranial Fossae. The upper surface of the base of the skull is divided into 

 three fossse, of which the cerebrum occupies the anterior and middle, whilst in the 

 posterior is lodged the cerebellum. 



The anterior fossa is defined posteriorly by the sharp, thin edge of the small 

 wings of the sphenoid, which curve laterally and slightly upwards, as well as back- 

 wards, to reach the region of the pterion laterally. The floor is formed from before 

 backwards, in the median plane, by the superior surface of the ethmoid and the 

 anterior part of the body of the sphenoid ; laterally it is constituted by the orbital 

 'parts of the frontal and the small wings of the sphenoid. On these the inferior 

 surface of the frontal lobes of the cerebrum rests. In front the fossa is divided 

 in the median plane by the frontal crest, to which the falx cerebri is attached. 

 This is confluent below with the anterior part of the crista galli, from which, 

 however, it is separated by the foramen caecum, which usually transmits a small 

 vein from the nose. On either side of the crista galli there are grooves which 

 vary considerably in depth and width: therein are lodged the olfactory bulbs. 

 The floor and sides of the groove are pierced by numerous foramina ; of these the 

 largest number transmit the olfactory nerves from the nasal cavity. In front an 

 elongated slit, placed on either side of the crista, affords a passage to the nose for 

 the anterior ethmoidal branch of the naso-ciliary nerve and a small branch of 

 the anterior ethmoidal artery which accompanies it. To the lateral side of the 

 olfactory groove and the cribriform plate, the anterior fossae communicate on either 

 side by means of the two ethmoidal foramina with the cavities of the orbits. The 

 anterior foramen transmits the anterior ethmoidal nerve and the anterior ethmoidal 

 artery ; the posterior affords passage to the posterior ethmoidal artery and nerve 

 (O.T. spheno-ethmoidal nerve of Luschka). Lateral to the olfactory groove, the 

 floor of the fossa, which here corresponds to the roof of the orbit, is very thin, as may 



