In this case the brain or a portion of it projects outside the skull (see Figs. 7 and 8). There may be distension of the ventricles in the extruded part (Hydrencephalocele) or there may not. It arises by dropsy of the ventricles without rupture.
In the more extreme cases (as in Figs. 7 and 8), the cranial bones are flattened down so as to form a very diminutive cranial cavity, while the brain, sometimes nearly of full size, lies outside, communicating with the interior through an aperture in the bones. In this case there has been at an early period a partial dropsy, which has caused displacement of the brain, and there is often evidence of this in the presence of two or three vesicles in the diminutive cranial cavity. Cleland has shown that these vesicles sometimes represent dropsical olfactory lobes with infundibula, which have pushed backwards the cerebral hemispheres, the latter at the period concerned being of small size. In other cases the extruded brain is hydrocephalic, and sometimes there is little more than a sac containing fluid (sometimes called Meningocele).
Fig. 7. - Bncephalocele. (W. I. M).
The position of the protrusion is most commonly behind the occiput or at the root of the nose, constituting encephalocele posterior or anterior. In rare cases it is lateral, the protrusion being above, in front of or behind the ear. It is also rarely seen projectipg upwards, .or downwards into the sphenoid sinus, the nares or mouth.