THE.ENDOSKELETON: SKULL AND VISCERAL SKELETON 113 



the cavities of the two sides are connected by a passage in the roof of the skull. 

 The bones surrounding the tympanic cavity are bones ossified in the otic capsules, 

 which are thus seen to be closely knit into the structure of the skull. At the 

 posterior end of the skull in the median line is a rounded opening, the foramen 

 magnum, surrounded by the occipital group of bones. Below the foramen is a 

 rounded projection, the occipital condyle. At either side of the posterior end of 

 the skull is a very large cavity, the pterygoid fossa, continuous dorsally with the 

 orbits and infratemporal fossae. The lateral margins of the posterior part of 

 the skull are formed by a bar of bone, which constitutes the lateral boundaries 

 of the orbits and infratemporal fossae; this is called the infratemporal arcade. 



On the ventral surface of the skull identify: a small opening near the tip, 

 the anterior palatine vacuity; a pair of larger oval openings ventral to the orbits, 

 the posterior palatine vacuities; and the posterior nares, a pair of small openings 

 in the median ventral line posterior to the preceding. 



The smaller holes in the skull are nerve foramina through which nerves enter 

 and leave the brain. The brain in life occupies the very small cavity which may 

 be seen by looking into the foramen magnum. The elongated cavities occupying 

 the interior of the remainder of the skull and extending from the anterior to the 

 posterior nares are the nasal passages. It will be noted that the posterior nares 

 have been moved far posteriorly from the position in which they are found in 

 Amphibia. 



2. Membrane bones of the roof of the skull. — At the anterior end of the 

 dorsal surface just behind the anterior nares is a pair of long bones, the nasals. 

 At each side of their posterior ends is a prefrontal bone, and at each side of that 

 a small lacrimal bone. Prefrontal and lacrimal bones form the anterior rim 

 of the orbit. The adlachrimal or supraorbital is a small bone lying loose in the 

 eyelid (and hence missing in many skulls) lateral to the prefrontal. Between 

 the two orbits dorsally is the frontal bone, single in the adult but double in the 

 embryo. In the median line posterior to the frontal is the parietal, also a single 

 bone in the adult but paired in the embryo. The postfrontal bone includes the 

 anterior part of the supratemporal fossa and sends down a process which forms 

 the upper half of the postorbital bar. Directly behind the postfrontal is the 

 squamosal which overhangs the external auditory meatus. 



3. The bones of the upper jaw. — The upper jaw consists of the original 

 cartilaginous half of the mandibular arch (pterygoquadrate cartilages) plus a 

 number of membrane bones. Although separate in elasmobranchs, it is insep- 

 arably fused to the skull in land vertebrates. It consists of two parts, a maxillary 

 arch forming the lateral parts of the skull both above and below and composed 

 entirely of membrane bones, and a median region developed from the pterygo- 

 quadrate cartilage, and consisting partly of cartilage bones and partly of mem- 

 brane bones. From the dorsal view identify the bones of the maxillary arch as 

 follows: premaxillae, in front of and at the sides of the anterior nares and bearing 



