266 VERTEBRATE ZOOLOGY 



In the hinder end of the skull is the foramen magnum, a large 

 circular opening through which the spinal cord enters the brain 

 cavity. The prominent, transverse lambdoidal ridge marks the 

 dorsal border of the hinder end, from the mid-dorsal point of which 

 the sagittal ridge runs forward. On each side of the foramen mag- 

 num are the occipital condyles, a pair of protuberances which 

 articulate with the atlas. Lateral to each condyle is the jugal 

 process. At the side of the skull, just back of the zygomatic arch, 

 is the external auditory meatus, the outer opening of the ear ; it 

 leads into the prominent, rounded tympanic bulla, which contains 

 the middle ear. At the anterior end of the skull are the nasal 

 capsules, the walls of which are partly cartilaginous, and the upper 

 and lower jaws. The opening of the nostrils, or external nares, 

 are at the forward end of the nasal capsules. The upper jaw 

 extends posteriorly to the orbits and bears the upper incisor, 

 canine, premolar, and molar teeth. The lower jaw forms the whole 

 ventral portion of the skull and bears the lower incisor, canine, 

 premolar, and molar teeth. Between the two sides of the lower 

 jaw and embedded in the floor of the pharynx is the hyoid 

 bone, which supports the tongue. It has already been described 



(P. 237)- 



On the ventral surface of the skull, on each side, is the promi- 

 nent tympanic bulla, which contains the middle ear. Between the 

 orbits is the opening of the nasopharynx. Just back of the incisor 

 teeth are two large openings, the foramina incisa. Note the articu- 

 lation of the lower jaw. 



Remove the lower jaw and study the bones of the skull as they 

 appear on the dorsal, the lateral, and the ventral surface. 



The bones of the cranium, as already suggested, fall into two 

 categories : those of the cranium proper, or brain case, and those 

 of the special-sense capsules. Those belonging to the brain case 

 occupy the central portions of the skull. The organs of hearing, 

 sight, and smell are lodged, in the embryo, in protective mem- 

 branous or cartilaginous capsules ; in the adult, certain bones have 

 developed in these capsules, and form an intimate union with 

 the brain case. The auditory capsule has undergone the most com- 

 plete ossification, the bones present in the adult forming the side 



