(_d_) _The side of the cranium._ At the base of the posterior end is seen the deep =tympanic cavity=. The dorsal part of this is divided by a vertical partition into two halves; of these the more anterior is the larger, and forms a deep funnel-shaped cavity, the =posterior opening of the Eustachian canal= (fig. 60, B, 26). A bristle passed into this opening emerges through the anterior opening of the Eustachian canal. The more posterior of the two is the =fenestral recess= (fig. 60, B, 28), and is in its turn divided by a slender horizontal bar into a dorsal hole, the =fenestra ovalis=, and a ventral hole, the =fenestra rotunda=. During life the fenestra ovalis lodges the proximal end of the =columellar= chain. Lying at the outer side and slightly dorsal to the tympanic cavity is a deep depression, the =lateral tympanic recess=, and immediately in front of this is the articular surface for the quadrate. The tympanic cavity is bounded below by the basitemporal, posteriorly by the exoccipital, and above by the _squamosal_, a membrane bone, which roofs over a good deal of the side of the cranium, and bears ventrally a prominent surface with which the quadrate articulates. Just in front of this is a large round hole, the =trigeminal foramen= (fig. 60, B, V), behind which the squamosal is drawn out into a short process. In front of the squamosal there is a prominent forwardly-projecting =postfrontal process= (fig. 60, 8), which ossifies from a different centre from that forming the squamosal, but in the adult is completely fused with it. The =orbit= forms a large more or less hemispherical cavity which lodges the eyeball. It is separated from its fellow of the opposite side by an imperfect partition, the =interorbital septum=. In the young skull it is seen to be bounded above by the frontal, with which the _lachrymal_ (fig. 60, 7) is fused anteriorly, forming a large backwardly-projecting process; while behind it is bounded by the =alisphenoid=. The interorbital septum is formed by the ossification and coalescence of the =mesethmoid= in front, with the =orbitosphenoid= behind, and the =rostrum= below. The boundary of the orbit below is very imperfect, the zygomatic arch being incomplete. The interorbital septum is pierced by the very prominent =optic foramen= (fig. 60, B, 2), just behind which are the two much smaller foramina for the exit of the oculomotor and pathetic (fig. 60, B, IV) nerves, the more anterior being that for the oculomotor. Above and slightly in front of the optic foramen is a median opening, the =olfactory foramen.= This leads into the cranial cavity behind, and in front is continued forwards as a groove between the interorbital septum and the frontal. (2) THE FACIAL PART OF THE SKULL. This includes the olfactory capsule and associated bones, and the upper jaw.