SPINA BIFIbA Mechanism of Development and Pathological Anatomy. and hernia occur in other parts of the spinal canal, and these behave quite similarly to those of the cranium in regard to their pathologic anatomy as well as to their mechanism of development. These eases, however, are frequently accompanied by other deformities (club-foot, knee defor mities) which point to degenerative factors.
There are fissures in the bony structure at certain places of the vertebral column and smaller or larger portions of the contents pro lapse through the fissure. Sometimes they are enveloped by the external membrane; in other eases the fissure involves the skin and soft parts, leaving the spinal cord exposed.
Thus in some of the case* we have clinically a tumor covered by a thin, fine membrane like old muslin, from which fluid is constantly exuding; in other cases this is firmer, and in the lightest cases there is only a pedunculated cyst which indicates the seat of the trouble. In spina bifida occulta the skin is unchanged over the ominous spot and it is only by defects in the nerve supply that a deformity at this place can be suspected.
The fissure formation in the vertebral column and the injury to the medullary canal are common to all these cases. They differ, however, as to the degree of the injury and the processes of repair which have taken place during the period of embryonal development.
In its first biological causes their origin is obscure, while their mechanism of development is easily intelligible from a knowledge of the development of the medullary canal.
The first stage of development of the spinal cord is a flat plate, and this descendant of the ectoderm later deepens in the centre forming a groove which slowly forms a tube and becomes detached from its matrix, the external blastoderm. The cho•da do•salis is previously formed from the middle blastoderm at the ventral side, and at both sides of this formation grows the cartilaginous and later osseous vertebral colunm, with the body enclosing the cord and medullary canal. The canal does not close simultaneously at all places; the upper cervical and the lower lumbar sections conclude the process (places of predilection for fissure formations).
According to the. degree of fissure formation we distinguish 1. Rachischisis or Fissure of the Spinal the arrest of
development occurs in the natural course of development at a time when the spinal cord is still flat and embedded without a groove in the ecto derm, it will remain in place, freely exposed, as a flat plate. The accu mulation of fluid in the arachnoidal space behind the medullary plate will push it forward, and, so to speak, turn it inside out.
In the centre there is a velvety, highly vascular mass, often with a central groove-like depression area medullo-vasculosa—pure medullary substance). A little away from the centre the covering commences with its soft meningeal membranes (area epithelio-serosa); then follows the sharply demarcated and highly vascular red-looking skin (area der matica).
In some rare eases the fissure also involves the anterior surface of the vertebral bodies: raehischisis anterior (Marchand).
2. exposed medullary plate referred to above may become detached from the rest of the body surface by the fluid behind it. and become attenuated to such an extent that it appears like the wall of a cyst, hut still remains discernible as medulla. The transparent sac is filled with fluid and the nerves run through its wall to the area mcdullo-vasculosa.
'Phis condition may undergo a change in that the zonular formation appears blurred; epithelial layers stretch across it and often there is only a cicatricial change at the top of t he sac which, together with the beginning of the nerve fibres, shows that we are dealing with the degen erated spinal cord and that the tumor signifies a prolapse of the entire cord. This is myelomeningocele.
This form also shows great defects of innervation; paralysis of the muscles which have their centre ill the destroyed part is a necessary sequence. (Paralysis of t he lower extremities, of the pelvic muscles, of the bladder, and of the rectum.) 3. Myelocystocele.—This form of spina bifida arises at a later period when the medullary canal is closed. The protrusion is forced outward by the increased pressure from within. There is an arrest of growth of the vertebral column, and the rapidly growing spinal cord not having enough room in the canal pushes through the back of the vertebral canal.