202 VERTEBRATE ZOOLOGY 



stapes articulates with the columella, the outermost and larger 

 of these ossicles, which is a rod of bone and cartilage extending 

 to the inner surface of the tympanic membrane. At the anterior 

 end of the tympanic cavity is the opening of the Eustachian tube, 

 which places the cavity in communication with the pharynx. 

 Probe it with a bristle. 



Projecting forward from the anterior border of the squamosal 

 are two short processes, the more dorsal being the zygomatic 

 process ; the more ventral articulates with a thick, irregular bone, 

 the quadrate. This bone is the suspensorium of the jaws on each 

 side; that is, it connects them with the cranium. Both upper and 

 lower jaws articulate with the ventral and anterior surfaces of it, 

 as will be seen later. The quadrates are not tightly joined with 

 the remainder of the skull but articulate freely with it. The 

 jaws are thus joined with the cranium by a double articulation 

 on each side. 



In front of and above the tympanic cavity and medial to the 

 quadrate bone is the trigeminal foramen, through which the tri- 

 geminal nerve passes. 



The orbit is a very large cavity which is bounded behind by 

 the squamosal and the alisphenoid, the latter bone being in front 

 of the former. Above, it is bounded by the large frontal bone, 

 which forms the whole roof of the skull between the orbits; in 

 front, by the frontal and the lachrymal, the latter bone forming 

 its anterior border. The inner wall of the two orbits, the inter- 

 orbital septum, is formed by the fusion of the presphenoid and 

 orbitosphenoid posteriorly, the elongated rostrum ventrally, and 

 the mesethmoid anteriorly. The ventral wall of the orbit is almost 

 entirely lacking. 



In the hinder part of the orbit is the large optic foramen through 

 which the optic nerve enters it ; just back of this are the foramina 

 of the oculomotor and trochlear nerves. 



Anterior to the frontals are the nasal and premaxillary bones, 

 which have already been described. 



Extending back on each side from the premaxilla to the quad- 

 rate is a slender bar, composed of three bones fused together, the 

 hinder part being the quadratojugal, the middle part the jugal, 



