PRENATAL GROWTH OF SPINAL CORD 43 



OBSERVATIONS AND DISCUSSION 

 A. CORD AS A WHOLE 



1. General form of the prenatal spinal cord {figs. 1 to 12; 

 tables 2 to 6) 



The spinal cord in the human embryos studied shows vari- 

 ous points of resemblance in form to the cord in the adult, as 

 shown in figures 6 to 12. Passing caudad from the brain there are 

 indications, however, that the embryonic spinal cord diminishes 

 gradually in its diameters to about the region of the 3rd cervical 

 segment. This resembles the cord of the child (Stilling), but not 

 of the adult. From here there is in the embryonic cord a slight 

 but constant increase in caliber, which reaches its maximum in 

 the region of the 4th or 5th cervical segment. This enlargement 

 in the cervical region corresponds to the intumescentia cervicalis. 

 This is followed by a more gradual decrease in area of cross-section, 

 which usually extends to about the 3rd or 4th thoracic segment. 

 The thoracic segments, from the 3rd to the 8th, have practically 

 the same area of cross-section in each of the embryos examined. 

 This is also true in the child, but to a less extent in the adult. 

 Near the lower end of the thoracic region there are indications of 

 the intumescentia lumbalis. The cord in general increases in area 

 of cross-section from the lower thoracic segments to the 5th lumbar 

 segment or thereabout, whence there is a gradual tapering of the 

 cord as it decreases in cross-sectional area to its end in the filum 

 terminale. The lower ends of the cords are, therefore, somewhat 

 conical in shape. 



In general shape the prenatal cord in cross-section is at first 

 oval, being compressed in its transverse diameter (figs. 1 to 5). 

 Later, however, it becomes compressed in the dorso-ventral diam- 

 eter and expanded laterally. In the thoracic region, the spinal 

 cord more nearly retains a cylindrical form. In these respects the 

 later fetal cord approaches the postnatal condition. If we com- 

 pare the increase in cross-sectional area at successive stages with 

 the increase in volume of the spinal cord, we find in general that 

 the volume increases more rapidly. That is, the cord becomes 



