PRENATAL GROWTH OF SPINAL CORD 49 



period and the second year of postnatal life the cross-sectional 

 area of the thoracic region increases approximately 1000 per cent, 

 while from here to the adult there is an increase of only about 

 100 per cent. 



c. Lumbosacral region. The lumbo-sacral region of the spinal 

 cord shows (table 1) approximately the same rate of growth in 

 volume as the thoracic region and a faster one than the cervical 

 region up to the 17 mm. embryo. The lumbo-sacral region forms 

 31 per cent of the entire cord in the 11 mm. embryo. It increases 

 relatively until in the 31 mm. specimen, it comprises 38 per cent 

 of the whole cord. The lumbo-sacral region decreases in relative 

 size in later prenatal life until in the child and in the adult it 

 forms only 18 per cent of the entire cord. The sacral region 

 decreases relatively more than the lumbar. After they can be 

 differentiated (31 mm.) the lumbar region is about seventeen 

 times as large as the sacral. In the child, however, the lumbar 

 region is twice as large as the sacral, and in the adult nearly four 

 times as great. 



The 4th lumbar segment is used to compare the areas of cross- 

 section in the lumbar region (figs. 1 to 5). In the younger em- 

 bryos (11 mm. and 17 mm.) this segment could not be measured 

 owing to the curvature of the cord. In the 65 mm. embryo the 

 cross-sectional area of the lumbar region is much larger, relatively 

 and absolutely, than the cervical region, as previously stated. 

 The rate of growth of the lumbar region in area of cross-section 

 is slower than that for the cervical or thoracic regions in the 65 

 mm. and 150 mm. embryos. This same relative decrease, hke 

 that for the volume, continues in this region in the child and 

 adult. 



This relative decrease in the lumbar and sacral regions is sur- 

 prising, for it seems that since the corresponding parts of the 

 body (the pelvis and lower extremities) increase in relative size 

 during this period, we should expect a relative increase in this 

 region of the cord. This decrease is evidently associated with 

 the shortening of the spinal cord within the vertebral canal which 

 begins about the third month and results in the well known 

 retraction of the lower end of the cord. 



