78 



Anatomy of the; Rabbit. 



relations of which are more fully dealt with below (p. 90). The 

 dorsal portion of the tympanic bulla is continuous with a short bony 

 tube which opens at a short distance dorsad by a large oval aperture. 

 This tube is part of a more extensive canal, the external acoustic meatus 

 (meatus acusticus externus) which, in the natural condition, leads down- 

 ward through the base of the external ear to the tympanic membrane. 

 The tvmpanic bulla does not form the lateral wall of the skull in this 

 region, and is not exposed to the cranial cavity. It is applied closely to 

 the external surface of the periotic or petromastoid bone, which forms 

 the lateral boundary of the cranial cavity, and contains the structures of 

 the internal ear. The external 

 or mastoid portion of this bone 

 appears in the space enclosed 

 between the tympanic bulla and 

 the jugular process of the occi- 

 pital bone, w^here it is readily 

 distinguishable by its pitted 

 appearance. Its ventral portion 

 bears a slender projection, lying 

 parallel to the jugular process, 

 the mastoid process (processus 

 mastoideus). 



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, put the cranial 

 cavity in communication with 

 the outside, and serve for the 

 passage of nerves and vessels. 

 The first and largest of these, 

 the optic foramen (foramen 

 opticum), occupies the middle 

 portion of the orbit, and trans- 

 mits, in the natural condition, 

 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 



Fig. 31. Dorsal surface of the skull: F, frontal; 

 I, interparietal; L, lacrimal; M. maxilla; JVIS, 

 mastoid poriion of petrosal (petromastoid); N, 

 nasal; P, parietal; SO, supraoccipital (squamous 

 portion of occipital); SO, squamosal; ZY, 

 zygomatic. > 



f.m,x.,' maxillary fossa; f.t., temporal fossa; 

 l.n.s.. superior nuchal line; l.t., temporal line; 

 p.f., frontal process of prema^illa; p.mx., 

 maxillary process of frontal; p.o.e., external 

 occipiLal protuberance; p.s.a. and p.s.p., 

 anterior and posterior supraorbital processes of 

 frontal; p.sc, subcutaneous process of lacrimal' 

 p.z., zygomatic process of squamosal; p.z.m.., 

 zygomatic process of maxilla: s.f., frontal spine, 

 s.m., spina masseterica. 



