Exencephalic ;lotuses.
These constitute a class characterized by the presence of a badly-formed brain, in part external to the skull which is itself imperfect. It is a transition stage, so to speak, between the celosomic and the acephalic monsters. Two varieties exist according as the fissure involves the verte bral column or not, a fissure which must not, however, be confounded with that which is characteristic of spina bifida.
Exencephalic foetuses without spinal fissure include the notencepbalic, proencephalic, podencephalic, hyperencephalic foetuses of Saint-Hilaire.
Exencephalic foetuses with spinal fissure include the iniencephalic and the exencephalic foetuses of the same authority.
Notencephalic Fcetuses.—Where the brain is almost entirely outside of the cranial cavity and posteriorly. The occipital portion of the skull is open, but there is no spinal fissure. At the level of the nucha is found a tumor with a pedicle, reposing on the neck and upper portion of the back, but not adherent. The volume of the tumor depends on the amount of cerebral matter which it contains and the concomitant hydrocephalus.
The point of exit of the tumor is at the junction of the superior and posterior part of the occipital bone (Fig. 188). These foetuses usually die in a few days.
Proencephalic Fcetuses.—Where the brain herniates anteriorly through an opening in the frontal bone. It is rarer than the preceding. The monstrosity may exist alone, or be accompanied by deformities of the face, etc. The vault of the skull is flattened.
Podencephalic Pcetuses.—Where the hernia of the brain is from the vault of the skull (Fig. 190). Ordinarily the tumor is circular, and may involve the sagittal, fronto-parietal sutures, etc.
Hyperencephalic Fa3tuse8.—Where the upper portion of the skull is almost entirely lacking, the bones of this region existing only as rudimen tary pieces. The frontal bones are long narrow shells, the parietal bones are stretched horizontally along the upper border of the temporals. The entire portion of the occipital is rudimentary in its upper part, the basilar apophysis being well developed. It is in reality a highly exaggerated stage of the preceding variety. According to Saint-Hilaire, these foetuses are nearly always of the male sex, often premature, and are not viable.
Iniencephalic Fcetuses.—Consist in hernia of the brain at the occipital bone with vertebral fissure. (Fig. 191.) They are really hence noten cephalic with fissure of the spine. The vertebral canal is open through out nearly its whole extent, and the malformation here is nearly if not equally as marked as that in the skull (Fig. 192.)
As Duges says " the head is confounded with the thorax and thrown so far forward that the occiput appears lost between the shoulders." The tumor contracts adhesions with the membranes which cover it. The quantity of cerebral matter outside of the skull varies.
Exencephalic Fceluses, properly so called, are characterized by the fact that the brain is almost entirely external, and posteriorly associated with spinal fissure. They are similar to the hyperencephalic with the fissure of the vertebral canal superadded. The upper portion of the skull is en tirely lacking, whilst in iniencephalic fcetuses there simply exists an opening in the occipital bone. (Fig. 192.) The herniated mass may be so large as to rest upon the back.
In general, hernia of the brain at the base of the skull is rare. In 93 cases collected by Houel only 9 were of this nature. The seat of the her nia varies. Usually it is at the cribriform plate of the ethmoid; again at the sutures which unite the frontal bone to the nose or the superior illa. At times the mass fills both orbits, (Fig. 193). Again the opening is at the Bella turcica (Fig. 195.) In a case cited by Ahlfeld and in Vir chow's, the tumor had perforated the base of the skull and the palate, and projected through the mouth.
Pseudencephalic Fetuses.—These fcetuses, according to Saint-Hilaire, do not possess any brain, properly speaking, since the nerve substance has almost entirely disappeared. The vault of the skull is almost en tirely lacking. The tumor is deep red in color, and made up of interlal ing small vessels filled with blood, and separated by debris of brain sub stance. The mass has a similar texture to erectile tumors. These fcetme are eminently typical in appearance. " The head has neither forehead nor vertex, is sunk between the shoulders and surmounted by a blood tumor. The face is much developed, directed obliquely, almost always livid. The hair is scanty but long and encircling the tumor. The nom is large and flattened; the mouth usually open; the eyes large and pro jecting, and at the top of the head; the ears deformed and often hanging down. These monsters have a truly hideous and unnatural appearance.' (Saint-Hilaire.) The vertebral canal is only slightly altered or normal. The spinal marrow exists. These fetuses are usually of the male sex, and can hardly be considered as able to survive beyond a few hours.