THE SKULL AS A WHOLE 17r 



ral condition, leads downward through the base of the external 

 ear to the tympanic membrane. The tympanic bulla is not 

 exposed to the cranial cavity. It is applied closely to the external 

 surface of the periotic or petromastoid bone (os petrosum), 

 which forms the lateral boundary of the cranial cavity, 

 and contains the structures of the internal ear. The exter- 

 nal or mastoid portion of this bone appears in the space enclosed 

 between the tympanic bulla and the jugular process of the oc- 

 cipital bone, where it is readily distinguishable by its pitted ap- 

 pearance. Its ventral portion bears a slender projection, lying 

 parallel to the jugular process, the mastoid process (processus 

 mastoideus) which is the point of insertion of one of the neck 

 muscles (sternomastoid). 



A series of foramina, lying partly within the orbit and extending 

 thence posteriorly along the boundary between the lateral and 

 ventral walls to the occiput, puts the cranial cavity in communica- 

 tion with the outside, and serves for the passage of nerves and vessels. 

 The first and largest of these openings, the optic foramen (foramen 

 opticum ) , occupies the middle portion of the orbit, and, in the natural 

 condition, transmits the optic nerve. Following this is a vertical 

 slit-like aperture — not to be confused with the perforations of the 

 external lamina of the pterygoid process — the superior orbital 

 fissure (fissura orbitalis superior). It represents both the superior 

 orbital fissure of the normal mammalian skull and the foramen 

 rotundum, and provides for the passage outward of the third, 

 fourth, and sixth cranial nerves, together with the first and second 

 divisions of the fifth. The lateral lamina of the pterygoid process 

 presents three foramina, of which the largest, anterior, and medial 

 one, the anterior sphenoidal foramen (alar canal), serves for the 

 transmission of the internal maxillary artery on its course dorsad 

 into the orbit, while the remaining two, the middle and posterior 

 sphenoidal foramina, transmit dorsally-directed muscular branches 

 (massetericotemporal and pterygobuccinator) of the mandibular 

 nerve. On the medial side of the base of the medial lamina of the 

 pterygoid process there is a shallow longitudinal groove, represent- 

 ing the pterygoid canal (canalis pterygoidus) of the human skull. 

 This accommodates a nerve (the Vidian, from the facial and the 

 sympathetic). Immediately in front of the tympanic bulla, on 



