60 



DEVELOPMENT OF THE SPINAL CORD. 



increase in extent. The anterior fissure is simply a clef t left between the enlarging 

 lateral halves of the cord ; the anterior commissure is formed across the bottom of 

 the cleft, which is thereby separated from the central canal. As for the posterior 

 fissure, it is uncertain whether it is in part formed from the dorsal portion of the 

 constricted canal, which has become occupied by an ingrowth of pia mater, and 

 converted into a mere septum of connective tissue, or whether this fissure with its 

 connective tissue septum becomes formed independently of the central canal, which, 

 as the fissure extends, gradually atrophies until it is eventually converted into the 

 rudimentary epithelial tube which is persistent during life. 



In the sacral region of birds, the central canal expands into the rhomboidal sinus, and in 

 the ftlum terminale of the human cord it remains relatively large. An open enlargement 

 analogous to the rhomboidal sinus of birds, although relatively smaller, has been described by 

 Tiedemann in a nine-week human foetus. 



The cord is at first oblong oval in section, with an angular depression in each 

 side which serves to mark off the situation of the future posterior columns and 

 their corresponding grey matter from the antero-lateral region. These two parts of 

 the lateral neural epiblast may be distinguished as the dorso-lateral (alar) and the 

 ventro-lateral (basal) laminae ; with the former, the afferent nerve-fibres become 



Fig. 67. BRAIN AND SPINAL CORD EXPOSED FROM BEHIND IN A FOETUS OF 

 THREE MONTHS (from Kolliker). 



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

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

 tricle, with remains of the membrana obturatoria. The spinal cord, s, extends to 

 the lower end of the sacral canal, and shows brachial and crural enlargements. 



connected, whilst from the latter the efferent fibres take origin 

 (His). In the human embryo of six weeks, they are well marked 

 off from one another, and their respective connections with the 

 posterior and anterior nerve-roots are very distinct (fig. 66). In 

 the upper part of the cord, the lateral nerve-roots (spinal accessory) 

 also arise from the basal lamina. The characteristic cylindrical 

 form of the cord is only attained with the development of the 

 lateral columns. The cervical and lumbar enlargements are mani- 

 fest at the end of the third month. 



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 

 filum 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 

 parts corresponding with the tracts of conduction ; the last of these tracts to become 

 medullated are the pyramidal tracts. 



The membranes are formed from mesoblast of the pro to vertebrae, which extends 

 over and under the cord, and becomes enclosed along with that structure within 

 the developing vertebral canal. The septa of connective tissue which are seen 

 penetrating into the substance of the cord from the pia mater grow in from this 

 mesoblast, carrying blood-vessels amongst the nervous elements. The neuroglia or 



