534 THE THORAX AND VERTEBRAL COLUMN 



The mesoderm which surrounds the neural tube forms the 

 vertebral column and the membranes of the brain and the 

 spinal medulla. Two bars of cartilage appear in each segment, 

 one at each side of the neural tube,, and grow backwards to form 

 the vertebral arch. They fuse with one another dorsally during 

 the fourth month and enclose the neural tube, surrounded by 

 a sheath of mesoderm ; which becomes differentiated into the 

 dura mater, arachnoid, and pia mater. 



The cartilaginous bars may fail to fuse on the dorsal aspect 

 of the neural tube. This condition, which is termed spina 

 bifida, most commonly occurs in the lumbo-sacral region and 

 may affect several consecutive segments. It is usually 

 associated with anomalies of the spinal medulla and its 

 membranes, but it may occur alone spina bifida occulta. 



The anomalies associated with spina bifida are subdivided 

 into several varieties, and the precise nature of the anomaly can 

 rarely be determined until the sac of the tumour which it forms 

 is opened. 



1. In a Meningocele the spinal medulla is normal in 

 development and position, and it possesses a complete covering 

 of pia mater. The arachnoid and the dura mater form a hernia, 

 filled with cerebro-spinal fluid, which projects backwards through 

 the deficiency in the vertebral arches and forms a tumour under 

 the skin of the back. Sometimes the dura mater is deficient 

 dorsally and the sac of the tumour is formed solely by the 

 arachnoid (Fig. 164, a) ; at others, the arachnoid maintains 

 its normal relationship to the spinal medulla and the sac is 

 formed by the dura mater alone. 



2. The Myelo-Meningocele differs from a meningocele in 

 that the spinal medulla, though normally developed and covered 

 by pia mater, herniates backwards into the sac of the tumour. 

 Both the spinal medulla and the nerve-roots arising from it may 

 be adherent to the inner wall of the sac (Fig. 164, b). 



3. In a Myelo-Cystocele the central canal of the spinal 

 medulla is enlarged in a backward direction, and its thin dorsal 

 wall, which retains its primitive embryonic structure, projects 

 through the deficiency in the vertebral arch. The dura mater 

 is said to be completely absent over the dorsal surface of the 

 tumour and the arachnoid is in contact with the skin (Fig. 164, c). 



4. In a Myelo-Cysto-Meningocele the condition is similar 

 to (3), but, in addition, the subarachnoid space is greatly dilated 

 and forms an arachnoidal meningocele. 



