782 THE NERVOUS SYSTEM. 



orbit. Over this is an irregular body representing the nose. The rest of the face is 

 well formed. The eyeball may be wanting entirely, or there are two eyes joined to- 

 gether, or, more seldom, two separate eyes. The orbit is surrounded by rudiments of 

 four eyelids. The frontal bone is single, the nasal bones are undeveloped; the ethmoid, 

 vomer, and turbinated bones are absent. The optic nerve is double, single, or absent. 

 There may be hydrocephalus. Such children are incapable of prolonged existence. 



ANENCEPHALIA. This malformation may be of various degrees. The brain may 

 be entirely absent, the base of the cranium being covered with a thick membrane, into 

 which the nerves pass. The membranes may form a sort of cyst containing blood and 

 serum, or portions of brain. Of the cranial bones, only those which form the base of 

 the skull are present (acrania). The scalp is usually partly or entirely absent over the 

 opening in the skull ; the eyes are prominent, and the forehead slopes sharply back- 

 ward. This malformation may occur in otherwise well-developed children. 



HYDROCEPHALUS. Tins lesion has been already considered above. It is proba- 

 ble that in some cases hydrocephalus intermts is due to a primary partial anencephalia, 

 and that the accumulation of fluid is of secondary occurrence. In rare cases, only 

 part of one lateral ventricle is hydrocephalic, giving to the head a protuberance on one 

 side. The viability of the fetus depends upon the degree of the hydrocephalus. 

 Hydrocephalus externus is an accumulation of serum beneath the pia mater, or, according 

 to some authors, between the pia and dura mater. It causes dilatation of the cranium 

 and compression of the brain. It is of very rare occurrence, and may also be secondary 

 to partial anencephalia. 



CEPHALOCELE, OR BRAIN HERNIA. When abnormal openings exist in the skull 

 from malformation, the contents of the cerebral cavity are apt to protrude in the form 

 of larger or smaller sacs. This may occur in cases of well-marked anencephalia or in 

 cases in which the brain is well developed. The protruding sac formed of the mcninges 

 may or may not be covered with skin. If the contents of the sac are simply fluid, the 

 lesion is called hydromeningocele ; if composed of brain substance, encephalocele ; if the 

 sac contain both fluid and brain substance, it is called hydrencepJialocele. The sacs may 

 be very small or as large as a child's head. They may protrude from the top of the 

 skull in acrania. They most frequently protrude through openings in the occipital 

 bone, often hanging down in large sacs upon the neck ; also at the root of the nose, 

 along the line of the sutures, at the base of the skull, and elsewhere. * 



MICROCEPHALIA. This is an abnormally small size of the brain, with a correspond- 

 ingly small cranium. The diminution in size affects principally the cerebral hemi- 

 spheres, though the other parts of the brain are also small. Thus the cerebellum may 

 be of extremely small size. The cord may be smaller than normal, in which case the 

 diminution in size is apt to show a direct dependence upon the cerebral lesions, the 

 direct and the crossed pyramidal tracts being most affected. There may also be present 

 imperfect development of the posterior columns and of the direct cerebellar tracts. The 

 convolutions are few and simple, the cavities often dilated with serum ; on the mem- 

 branes there may be traces of inflammation. The cranium is small, the face large, the 

 rest of the body small. The malformation is in some cases caused by inflammation or 

 dropsy of the brain during foetal life. It is endemic in some countries, buc single cases 

 may occur anywhere. The fetus is viable. Absence or incomplete development of 

 portions of the brain may occur, not only in idiots, but in persons whose minds are 

 perfect. 1 



Malformations of the Spinal Cord. 



The malformations of the spinal cord may be conveniently classed as follows (Van 

 Gieson 2 ) : 



I. CONGENITAL DEFORMITIES ASSOCIATED WITH MONSTROSITIES, AND INCOMPATIBLE 

 WITH EXTRA-UTERINE LIFE. 



1 For a general consideration of malformations of the central nervous system con- 

 sult Thoma, "Text-Book of Pathological Anatomy," vol. i., p. 206 et seq. 



5 Van Gieson, "Artefacts of the Nervous System," New York Medical Journal, vol 

 Ivi., pp. 337, 365, 421, 1892. 



