iones 01 



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pneumogastric, and spinal accessory nerves. The under surf; 

 the basilar process is in the roof of the pharynx, and may be explored 

 by the finger in the case of suspected fracture of the base of the skull. 

 The bone is developed by seven centres : four for the vertical part, 

 which blend at the occipital protuberance, one for each condylar part, 

 and one for the basilar process. 



The vertical part of the bone, as with those parts of the other 

 bones which form the wall and roof of the skull, is developed from 

 membrane, whilst the horizontal part, as is the case with those other 

 pieces which form the base of the skull, is ossified from cartilage. 

 This arrangement is to render the skull strong enough to protect the 

 basal ganglia during its passage through the pelvis. 



Cranium bifidum occurs when osseous union in the occipital region 



is incomplete, themembranes, 

 and perhaps some of the en- 

 cephalon, bulging backwards 

 through the median cleft. 

 (See Spina Bifida, p. 204.) 



The parietal ( paries, 

 wall). The convex surface is 

 marked by the curved tem- 

 poral ridge, above which 

 plays the aponeurosis of the 

 occipito-frontaljs, the tem- 

 poral muscle arising below 

 the ridge. Near the hinder 

 part of the superior border is 

 the parietal foramen, by 

 which a vein for the scalp 

 passes into the longitudinal sinus, accompanied sometimes by a twig 

 of the occipital artery. 



The internal surface is marked by cerebral convolutions, especially 

 those of the motor area, and is deeply grooved by branches of the 

 middle meningeal artery, which lead upwards from the anterior inferior 

 angle. The posterior inferior angle is grooved for the lateral sinus, 

 and along the superior border runs the shallow groove for the longi- 

 tudinal sinus and the falx cerebri. Near this groove are depressions 

 for the Pacchionian glands villous processes of the arachnoid ; 

 uninformed man might take these irregular nodules for tubercu 

 deposits. Some of them are deeply imbedded in the bone. 



The posterior border articulates with the occipital in the lambdoid 

 suture. In weakly infants who lie much in the supine position, the 

 occipital bone may sink in between the parietals to such an extent as, 

 according to some observers, to cause cerebral irritation. The articu- 

 lation of the posterior inferior angle with the mastoid bone is often 

 thinned (cranio-tabes) in syphilitic and rickety children, imparting. 



Occipital Meningocele. 



