THE SPINAL MEDULLA 533 



necessary to expose the spinal medulla in order to remove the 

 cause of pressure, when it is being compressed, or to divide the 

 posterior nerve-roots, in intractable neuralgia (the " crises " of 

 locomotor ataxia). 



A vertical incision is made in the median plane and is carried 

 down to the vertebral spines. The muscles attached to the 

 spines and laminae are elevated on both sides, and this step may 

 give rise to severe haemorrhage. The bleeding, however, may 

 be controlled by packing tightly with gauze and forcibly 

 retracting the muscular masses in a lateral direction. The 

 supra- and inter-spinous ligaments of the vertebra selected are 

 cut and the lamina on each side is divided with an osteotome. 

 The instrument is applied with its cutting edge at right angles 

 to the inferior border of the lamina. In this way there is no 

 danger of cutting into the root of the vertebral arch (pedicle), 

 and, if the instrument slips when the division of the lamina is 

 completed, the spinal medulla cannot be injured. 



After both laminae have been divided, the ligamenta flava 

 (p. 527) are cut across, and the vertebral arch may be removed. 

 Additional laminae can then be removed with bone forceps. 

 Some loose fatty tissue containing numerous small veins is 

 exposed on the surface of the dura mater. The haemorrhage 

 from these veins may be so great as to necessitate a two-stage 

 operation. 



The dura mater is incised, and as the arachnoid may be 

 injured at the same time, cerebro -spinal fluid may escape in 

 company with subdural fluid. To prevent too great an outflow 

 the head-end of the table should be depressed. 



When the operation has been completed, the cut edges of 

 the dura mater are carefully united (p. 536) and the thick muscular 

 flaps are replaced. 



Development of the Nervous System and the Vertebral 

 Column. During the second week of intra-uterine life, a 

 longitudinal furrow, termed the neural groove, appears on the 

 dorsal aspect of the embryo. The walls and floor of this groove 

 are formed by a thickening of the surface ectoderm. At a 

 slightly later stage the margins of the groove unite and it 

 becomes converted into the neural tube, which subsequently 

 gives origin to the whole of the nervous system. At first the 

 dorsal part of the neural tube is continuous with the surface 

 ectoderm, but the two soon become separated by mesoderm, 

 which grows in from each side. 



34 a 



