Skull and Hyoid 193 



The basi-sphenoid is a direct continuation forward of the basi-occiput in the adult, 

 and forms the roof of the two bony openings into the nasal cavities : the openings 

 are separated by the vomer, one of the face bones, which articulates by expanded alee 

 with the under surface of the body of the sphenoid. The outer wall of each opening 

 is made by a down-growing process that is descriptively part of the sphenoid, the 

 internal pterygoid plate, ending in a hooked hamular process turning outwards. Outside 

 each internal plate is an external pterygoid plate, placed more obliquely. The pterygoid 

 plates are to be looked on, from a descriptive point of view, as processes from the 

 sphenoid the internal one from the under surface of the side of its body and the 

 external one from the under surface of the great wing, which projects out from the 

 body on each side. On the outer side of the base of the external plate the great wing 

 can be seen projecting outward : this wing, if followed outward, is seen to widen 

 rapidly antero-external to the petrous bone and then to narrow again, so that a sharply- 

 pointed angle (Fig 161) of the wing projects back and fits in between the petrous 

 and squamous parts of the temporal : the squama turns round on to the base in front 

 of the external meatus to form the articular cavity for the lower jaw and the base 

 just in front of this. The angle of the great wing can be recognised from the presence 

 of the spine of the sphenoid on it. 



We can therefore speak of the petrous as extending forward and inward between 

 the basi-occiput and margin of the great wing of sphenoid, so that its apex tends to 

 fit in between the great wing and the basi-sphenoid, but does not reach these bones : 

 the interval between them is the foramen lacerum. 



The under or basal surface of the great wing can be followed back, as just seen, 

 to the angle between petrous and squamous, but can also be followed out to the inferior 

 temporal ridge. The ridge is a crest that runs across this great wing and also across 

 the squama behind it, and at the ridge the basal surfaces of these bones become con- 

 tinuous with their outer or temporal surfaces already described. 



The jugular foramen is bounded internally and behind by the occipital, externally 

 and in front by the temporal. Immediately to its outer side a long pointed styloid 

 process projects from the temporal in a downward, forward, and inward direction. 



Outside this the lower and front wall of the bony outer meatus is formed by the 

 tympanic plate, and this plate is separated in front by a narrow but deep Glaserian 

 fissure from the articular surface : the whole concavity is termed the glenoid cavity. 

 It is bounded in front by the eminentia articularis, a root of the zygoma which forms the 

 back limit of the passage under that arch. 



The temporal is a compound bone whose various parts will be dealt with subse- 

 quently, but its general divisions can be easily and advantageously made out now. 

 The petrous is all that part in contact with the occipital, and includes the basis of the 

 mastoid process. The squama forms the articular area, and base just in front of this, 

 as well as practically all the bone which appears on the lateral aspect of the skull, 

 including the zygoma ; but in the lower and back part of this surface the squama only 

 partly covers in the petrous basis of the mastoid, and thus a petro-squamous suture 

 is visible, behind and below, while the petrous bone is seen laterally (Fig. 160). The 

 styloid process and tympanic plate are morphologically separate portions of the bone, 

 which may therefore be said to be compounded of four parts not counting the ossicles 

 of the middle ear that are enclosed in it. 



A large round foramen in the basal surface of the petrous, in front of the jugular 

 foramen, is the opening of the carotid canal in that bone for the passage of the internal 

 carotid artery. 



Various foramina in the basal surface of the great wing will be studied later. 



F.A. ,3 



