40 OSTEOLOGY. 



Outside the palatine ridges there are two very large spaces, the 

 orbito-temporal cavities ; each of these is bounded externally by 

 the zygomatic arch, a horizontal process composed of parts of 

 th^*«*fl{SaT^'^n^ squamosal bones, anteriorly by the alveolar 

 tuberosity, posteriorly by the articular surface of the squamosal 

 bone, with which the inferior maxilla articulates, and internally 

 by the sphenoid and palatine bones. The cavities open each 

 by two large apertures on the lateral aspect of the skull ; the 

 posterior portion being called the temporal, the anterior the 

 orbital fossa. In the higher orders of mammals, the bimana 

 and quadrumana, these two fossce are completely separated from 

 each other by a bony process or plate. The articular surface 

 of the squamosal bone, above named, terminates posteriorly in 

 a projection called the anterior mastoid process, behind and 

 internal to v;hich is a very irregularly-shaped bone, the petrosal, 

 characterised by the external auditory meatus, a bony tube 

 which is directed outwards and upwards, and also by a slender 

 spike, projecting downwards and forwards, called the styloid 

 process : the petrosal bone contains the internal mechanism of the 

 ear, and also gives attachment to the cornu of the os hyoides, or 

 bone of the tongue. 



Between the petrosal and basi-occipital bones we have a large 

 and very irregularly-shaped aperture, leading into the cranial 

 cavity ; this is the forgi.pien lacerum basis cranii, and it gives 

 passage to some important nerves and vessels ; anteriorly, the 

 foramen lacerum is bounded by a flattened plate, the part of 

 the sphenoid bone called the wing or ali-sphenoid ; and on the 

 internal part of the edge of the wing is the carotid notch, through 

 which tbe endocarotid artery enters the cranium. Posterior to 

 the petrosal bones, two large processes point downwards ; these 

 are the occipital styloid processes, which must be distinguished 

 from the small petrosal ones before alluded to. Still farther back 

 we have the occipital condyles, by which the head articulates 

 with the atlas ; a deep notch, the stylo-condyloid notch, separates 

 each condyle from the styloid process, and at the bottom of the 

 notch on its internal aspect is the condyloid foramen, leading 

 into the cranium. 



The Lateral surfaces exhibit inferiorly the external face of the 

 rami of the inferior maxilla, and antero-superiorly a triangular 

 space, formed chiefly by the superior maxilla, more or less convex 

 but sometimes hollowed in old animals, and presenting the infra- 



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