Chap, ii.] BONY VAULT OF THE CRANIUM. 



parts of the skull as are formed in membrane, the 

 base remaining free. 



Among the more common of the gross mal- 

 formations of the skull also, is one that shows 

 entire absence of all that part of the cranium that is 

 formed in membrane, while the base, or cartilaginous 

 part, is more or less perfectly developed. 



Meningocele is the name given to a congenital 

 tumour that consists of a protrusion of a part of the 

 cerebral membranes through a gap in an imperfectly 

 developed skull. When the protrusion contains brain, 

 it is called an encephalocele, and when that pro- 

 truded brain is distended by an accumulation of fluid 

 within the ventricles, it is called hydrencephalocele. 

 These protrusions are most often met with in the 

 occipital bone, and next in frequency in the fronto- 

 nasal suture, while in rarer cases they have been met 

 with in the lambdoid, sagittal, and other sutures, and 

 have projected through normal and 

 abnormal fissures at the base of 

 the skull into the orbit, nose, and 

 mouth. Their frequency in the 

 occipital bone may be in some 

 way explained by a reference to 

 the development of that part. 

 This bone at birth consists of 

 four separate parts (see Fig. 3), 

 a basilar, two condylar, and a 

 tabular or expanded part. In the 

 tabular part, about the seventh 

 week of foetal life four nuclei 

 appear, an upper and a lower pair. 

 These nuclei are to some extent 

 separated by fissures running inwards from 

 four angles of the bone, and those that run in 

 from the upper and lateral angles persist for some 

 time after birth. Meningoceles of the occiput are 



Fig. 3. The Occipital 

 Bone at Birth. 



the 



