618 TEXT-BOOK OF EMBRYOLOGY. 



a small part merorachischisis, and is usually accompanied by curvature of the 

 spine. Sometimes the deformity of the vertebral column is continuous with 

 cranioschisis craniorachischisis. The more or less rudimentary spinal cord 

 lies along the bottom of the cleft. When the rachischisis is total the spinal 

 cord is practically wanting amyelus. 



Spina bifida is marked by the presence of a cyst which protrudes through a 

 cleft in the vertebral column; externally it presents the appearance of a sac- 

 like structure of variable size. Three different types of spina bifida may be 

 recognized, depending upon the structures involved. If the cord and its mem- 

 branes are included in the cyst it is known as myelomeningocele; if only 

 the membranes, as spinal meningocele; if the cord itself is dilated, as myelo- 

 cystocele. 



Myelomeningocele is the most common form of spina bifida and usually 

 occurs in the lumbo- sacral region, rarely in the cervical or thoracic region. 

 Its appearance is that of a rounded tumor in the medial line, and, if the child 

 lives, the tumor increases in size and may become as large as a child's head. 

 The spinal cord is bent dorsally and attached to the sac. The pia mater and 

 arachnoid surround the cord, while the dura is incomplete. The spinous 

 processes and the adjacent parts of the arches of the vertebrae are absent. 

 From one to several vertebrae may be affected. 



In spinal meningocele the spinal membranes bulge out to form a sac filled 

 with fluid. The vertebrae are not necessarily defective, for the sac may pro- 

 trude between the arches or through the intervertebral foramina; it more 

 often protrudes laterally than dorsally. The presence of meningocele is 

 not at all incompatible with life, but the sac usually enlarges to a good-sized 

 tumor. 



In myelocystocele (syringomyelocele) the central canal of the spinal cord is 

 dilated locally, with the result that a portion of the cord with the pia and 

 arachnoid becomes a cystic tumor. It may occur in any region, and is fre- 

 quently associated with asymmetrical defects of the vertebral column. 



Spina bifida occulta, a condition in which neither cleft nor tumor is visible 

 externally, is usually found in the lumbo-sacral region. The position of the 

 defect is indicated by a small depressed cicatrix or by a small tuft of hair. 

 The spinal cord is elongated and extends into the sacral canal. The spinal 

 canal is sometimes dilated and the cauda equina affected, in consequence of 

 which there are sensory and motor disturbances in the lower extremities. 

 Paralytic club-foot and derangement of the bladder functions may result 

 from such a deformity of the cord. 



Diastematomyelia, or doubling of the spinal cord, sometimes accompanies 

 rachischisis. The cord in such cases is represented by two atrophic bands. 



