Chap, ii.] BONY VAULT OF THE CRANIUM. 



remarkable to note that these changes are limited to 

 such 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. 



Mciiingoccle 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 fis- 

 sures at the base of the skull 

 into the orbit, nose, and mouth. 

 Their frequency in the occipital 

 bone may be in some way ex- 

 plained by a reference to the 

 development of that part. This 

 bone at birth consists of four 

 separate parts (see Fig. 3), a basi- 

 lar, 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 

 the four angles of the bone to meet at the occi- 

 pital protuberance. The gap running up in the 

 C 4 



Fig. 3. The Occipital 

 Bone at Birth. 



