1902 Encyclopedia > Abnormal Development & Congenital Malformations > Monstrosities from Defective Closure in the Middle Line

Abnormal Development and Congenital Malformations
(Part 6)

(6) Monstrosities from Defective Closure in the Middle Line

Under this head come some of the commonest congenital malformations, including slight deficiencies such as harelip, and serious defects such as a gap in the crown of the head with absence of the brain.

The embryo is originally a circular flattened disc spread out on one pole of the yolk, and it is formed into a cylindrical body (with four appendages) by the free margins of the disc, or rather its ventral laminae, folding inwards to meet in the middle line and so close in the pelvic, abdominal, thoracic, pharyngeal, and oral cavities. Meanwhile and indeed rather earlier, two longitudinal parallel ridges on the top or along the back of the disc have grown up and united in the middle line of form the second barrel of the body -- the neural canal -- of small and uniform width in the lower three-fourths of spinal regions, but expanding into a wide chamber for the brain. This division into neural (dorsal) and haemal (ventral) canals underlies all vertebrate development. Imperfect closure along either of those embryonic lines of junction may produce various degrees of monstrosity.

The simplest and commonest form, hardly to be reckoned in the present category, is harelip with or without cleft palate, which results from defective closure of the ventral laminae at their extreme upper end.

Another simple form, but of much more serious import, is a gap left in the neural canal at its lower end; usually the arches of the lumber vertebrae are deficient, and the fluid that surrounds the spinal cord bulges out in its membranes, producing a soft tumour under the skin at the lower part of the back. This is the condition known as hydrorhachis, depending on the osseous defect known as spina bifida, Children born with this defect are difficult to rear, and are very likely to die in a few days or weeks.

More rarely the gap in the arches of the vertebrae is in the region of the neck. If it extend all along the back, it will probably involve the skull also.

Deficiency of the crown of the head, and in the spine as well, may be not always traceable to want of formative power to close the canal in the middle line; an over-distended condition of the central water-canal and water-spaces of the cord and brain may prevent the closure of the bones, and ultimately lead to the disruption of the nervous organs themselves; and injuries to the mother, with inflammation set up in the foetus and its appendages, may be the more remote cause.

But it is by defect in the middle line that the mischief manifest itself, and it is in that anatomical category that the malformations are included. The osseous deficiency at the crown of the head is usually accompanied by want of the scalp, as well as of the brain and membranes. The bones of the face may be well developed and the features regular, except that the eyeballs bulge forward under the closed lids; but there is an abrupt horizontal line above the orbits where the bones cease, the skin of the brow joining on to a spongy kind of tissue that occupies the sides and floor of the cranium. This is the commonest form of an anencephalous or brainless monster. There are generally mere traces of the brain, although, in some rare and curious instances, the hemispheres are developed in an exposed position on the back of the neck. The cranial nerves are usually perfect, with the exception sometimes of the optic (and retina). Vegetative existence is not impossible, and a brainless monster has been known to survive sixty-five days. The child is usually a very large one.

Closely allied, as we have seen, to the anencephalus condition is the condition of congenital hydrocephalus. The nervous system at its beginning is a neural canal, not only as regards its bony covering, but in its interior; a wide space lined by ciliated epithelium and filled with water extends along he axis of the spinal cord, and expands into a series of water-chambers in the brain. As development proceeds, the walls thicken at the expense of the internal water-spaces, the original tubular or chambered plan of the central nervous system is departed from, and those organs assume the practically solid for in which we familiarly know them.

If, however, the water-which persist in their embryonic proportions notwithstanding the thickening of the nervous substance forming their walls, there results an enormous brain which is more than half occupied inside with water, contained in spaces that correspond on the whole to the ventricles of the brain as normally bounded. A hydrocephalic foetus may survive its birth, and will be more apt to be affected in its nutrition than its intelligence. In many cases the hydrocephalic condition does not come on till after the child is born.

The microcephalous condition, where it is not a part of cretinism, is not usually a congenital defect in the strict, but more often a consequence of the premature union of the bones of the skull along their sutures or lines of growth.

Returning to the ventral midline, there may be defects of closure below the lips and palate, as in the breast-bone (fissure of the sternum), at the navel (the lat point to close in any case), and along the middle line of the abdomen generally.

The commonest point for a gap in the middle line of the belly is at its lower part, an inch or two above the pubes. At that point in the embryo there issues the allantois, aballoon-like ezpansion from the ventral cavity, which carries on its outer surface blood- vessels the embryo to interdigitate with those of the mother on the uterine surface.

Having served its temporary purpose of carrying the blood-vessels across a space, the balloon-like allantois collapses, and rolls up into the rounded stem-like umbilical cord through most of its extent; but a portion of the sac within the body of the foetus is retained as the permanent urinary bladder. That economical adaptation of a portion of a vesicular organ, originally formed for purposes of communication between the embryo and the mother, appears to entail sometimes a defect in the wall of the abdomen just above the pubes, and defect in the anterior wall of the bladder itself.

This is the distressing congenital condition of fissure of the urinary bladder, in which it interior is exposed through an opening in the skin; the pubic bones are separated by an interval, and the reproductive organs are ill formed; the urachus is wanting, and the umbilicus is always placed exactly at the upper end of the gap in the skin. A monstrosity recalling the cloacal arrangement of the bird is met with as more extreme defect in the same parts.

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