Skull and Hyoid 227 



Pterygoid processes : the outer is developed in continuity with the great wing. 

 Each inner plate has a separate centre of ossification and fuses secondarily with the 

 mass of the bone. 



The centres for the great wing and internal plates appear towards the end of the 

 second month, those for the presphenoid and small wings early in the third month, 

 while the basisphenoid begins to ossify in the latter part of this month. The centre 

 for each lingula is the last to appear, during the fifth month. The bony parts of the 

 internal pterygoid plates and the two halves of the basisphenoid unite in the fourth 

 month, and are joined some time later by the lingulae. The orbito-sphenoids and their 

 corresponding presphenoid centres also unite on each side in the fourth month, but it is 

 not until the eighth month that the two halves of the presphenoid unite with each other 

 and the basisphenoid, and even at birth the fusion is not complete, but is represented 

 in part by a cartilaginous junction. In the dried specimen the situation of this 

 cartilage is shown by one or two centrally or laterally placed fossae or foramina, some- 

 times termed " cranio-pharyngeal canals," but they should not be confused with the 

 canal properly so called that may result from partial persistence of the track of Rathke's 

 pouch ; the latter is normally hardly distinguishable in the cartilaginous skull before 

 ossification, and should be completely obliterated by extension forward of the 

 basisphenoidal centres. 



Sphenoidal Turbinate Bones. 



These thin anterior coverings of the sphenoidal sinuses, often termed conches 

 sphenoidales or bones of Berlin, are found as a rule attached to the sphenoid, but as they 

 are developed as ossifications associated with the posterior cupola of the cartilaginous 

 nasal capsule, quite distinctly separate from the sphenoid, they are considered 

 separately here. Each bone fits as a cap on the front of the corresponding sinus and 

 presents a rounded opening above the centre of its anterior surface : the outer and lower 

 corner of this surface is prolonged outwards and downwards with a sharp angle, and 

 the whole surface is applied to the back of the ethmoid. 



The lower aspect is separated from its fellow by the projection of the rostrum, and 

 forms a point which fits in between the rostrum and the internal pterygoid plate. 

 The inner, upper and outer parts of the bone are applied to and fused with the sphenoidal 

 septum and upper and outer parts of the body of the sphenoid respectively. The lower 

 surface appears in the roof of the nasal fossa, and, outside this, articulates with 

 the palate bone and forms the upper boundary of the spheno-palatine foramen : 

 the outer surface is in the inner wall of the orbit between the ethmoid and the 

 presphenoid. 



Each bone is usually destroyed in part during disarticulation owing to its 

 adhesion to the neighbouring bones : a fairly complete specimen is shown on one side in 

 Fig. 183. 



Each ossicle commences to ossify in the fifth month by a centre on the innner part 

 of the cartilaginous cupola of the nasal capsule, followed about a couple of months 

 later by a second centre on the outer part. These centres form the bone by fusion 

 with each other and with additional lower centres in the first year or so of life. During 

 the fourth and fifth year the cartilaginous structure is absorbed, and this absorption 

 extends to the outer and back part of the bony structure, so that the commencing 

 sphenoidal sinus comes into relation with the sphenoid, into which it commences to 

 extend about the seventh year. The bones are not fused with the sphenoid until the 

 cartilage is absorbed. 



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