SPINAL CORD 



103 



The so-called posterior or dorsal median fissure appears first in the sixth week 

 as a distinct infolding of the dorsal plate towards the lumen. At this time the 

 primitive dorsal columns are still separated by the projection of the plate on 

 the dorsal aspect of the cord. In the succeeding weeks the upper part of the 

 primitive canal appears to be obliterated, and only the ventral portion persists 

 as the definitive central canal of the cord. The apparent closure of the lumen is 

 associated with great expansion of the primary dorsal columns (future tracts of 

 Burdach) and the formation of new mesial columns (future tracts of Goll) (fig. 139). 

 As these columns increase in size, the dorsal horns of the grey crescents are 

 isolated and clothed on their mesial aspects by white substance. Meanwhile the 

 walls of the dorsal part of the central canal become apposed, and the ependymal 

 cells become obliquely arranged, the obliquity of their direction increasing as the 

 dorsal wall is approached. From this a mesial fibrillar septum extends to the 

 free surface. These appearances suggest that the canal is not obliterated by a 

 simple fusion of its walls, but, on the other hand, they cannot be explained wholly 

 by the growth of the posterior columns and posterior horns with persistence of 

 the primary canal and extension of the primary infolding of the 

 dorsal plate. Both factors probably share in the process. 



The end part of the central canal is expanded into what 

 has been termed the terminal ventricle. It forms the dilated 

 part of the central canal in the conus medullaris. 



FIG. 140. BRAIN AND SPINAL COED EXPOSED FBOM BEHIND IN A FCETUS OF 

 THBEE MONTHS. (From Kolliker.) 



h, the hemispheres ; m, the mesencephalic vesicle or corpora quadrigemina ; 

 c, the cerebellum ; below this are the medulla oblongata, mo, and fourth 

 ventricle, with remains of the meinbrana obturatoria. The spinal cord, 

 s, extends to the lower end of the sacral canal, and shows brachial and crural 

 enlargements. 



In the sixth week the lateral walls show a distinct separation 

 into alar and basal lamince. With the former 'the afferent nerve- 

 fibres become connected ; while from the latter the efferent 

 fibres take origin (His). 



The characteristic cylindrical form of the cord is only attained 

 with the development of the lateral columns. The cervical and 

 lumbar enlargements are manifest at the end of the third month 

 (fig. 140). 



Up to the fourth month, the cord and the vertebral canal increase in length 

 pari passu, but the vertebral column then begins to grow more rapidly than the 

 cord, so that by the time of birth the coccygeal end of the cord is opposite the 

 third lumbar vertebra, while in the adult its limit is the lower end of the first 

 lumbar. Along with this relative shifting of the cord and its containing tube 

 the lower nerve-roots lose their regular rectangular course, and become oblique. 

 They alone, with the fttum terminate, occupy the lower end of the neural canal, 

 where they form the cauda equina. 



The nerve-fibres of the white columns are at first entirely non-medullated, and 

 the white substance has a greyish transparent appearance. The medullary sheath 

 is not formed simultaneously in all parts, but appears at different times in different 



1 In the figures published by His of the developing spinal cord, certain other folds of the wall are 

 represented. Wilson (Jour, of Anat. andPhys. vol. xl.) has recently again drawn attention to these folds. 

 He suggests that they have morphological significance, and may possibly indicate a triple division of 

 the neural tube. It is doubtful, however, how far such folds or furrows are to be regarded as normal. 

 The appearance of the basal plate (see fig. 138) seems to be correlated with the earlier differentiation of 

 the motor region of the mantle zone. In a well-fixed embryo of 15'5 mm. (obtained at an operation), 

 and in one of 21 mm. I cannot satisfy myself that there are any irregularities of the wall of th 

 such as are seen in His' figures. T. H. B. 



