This name implies that the brain is absent. It arises by a non-closure of the medullary canal or an early rupture of it.

The child is often born at the full time, and the trunk and limbs are usually well-developed. But the vault of the cranium is absent, and the base of the skull exposed (see Fig. 6). The base is occupied by loose membrane in which there may be some cysts. Occasionally there is a vestige of brain, with perhaps a small cavity communicating with the surface, in which case the early defect has been limited and has allowed of some development of the brain. The membrane at the base represents arachnoid and pia mater turned aside, with the ventricles exposed. In some cases there is a sac occupying the place of the brain, and representing the distended but unruptured ventricles. The absence of the cranial vault renders the eyes unduly prominent as they project at the edge of the open skull (see Fig. 5), giving the head the appearance of a toad, from which the malformation is sometimes called popularly Toad's Head.

Fig. 5. - Auencephalus. (W. I. M).

Fig. 6. - Anencephalus in median section. The cleft passing horizontally and downwards from right to left is the mouth and fauces, with tongue below and palate and nares above. The bodies of the vertebras are surmounted by the bones of the base of the skull seen in section. (W. I. M).

The cranium shows almost complete absence of the flat bones of the vault, a shortening of the base and an angular curvature between the sphenoid and occipital bones (see Fig. 6). In view of the open state of the cranium anencephalus is sometimes designated Cranioschisis or Acrania. The malformation is often combined with a similar lesion of the spine (spina bifida).