I050 



HUMAN ANATOMY. 



ing of the dorsal part of the canal in consequence of the approximation of its walls, which in the 

 course of the seventh week is closer and, by the end of the second month is completed by the 

 meeting and fusion of the adjacent inner layers, with obliteration of the intervening cleft 

 and the production of the posterior median septum in its place. Since the partition is formed 

 by the union of the inner (ependymal) layers, it is probable that the septum is to be regarded 

 as essentially neurogliar in origin and character. It must be remembered, however, that a 

 certain amount of mesoblastic tissue may be later introduced in company with the blood-vessels 

 which subsequently invade the septum. The remaining and unclosed part of the lumen for 

 a time resembles in outline the conventional spade of the playing card, with the stem directed 

 ventrally ; but later gradually diminishes in size and acquires the contour of the definite central 

 canal. 



During these alterations in the e.xtent and form of its lumen, the gray matter of the develop- 

 ing cord markedly increases, especially behind where the posterior horn appears as a projection 

 beneath the broadening mass of the ingrowing dorsal root-fibres. As the posterior horn becomes 

 better defined, the root-bundle becomes meso-laterally displaced, lying behind the horn, and 

 then constitutes the tract of Burdach. Goll's tract is formed somewhat later and at about the 

 third month appears as a narrow wedge-shaped area that is introduced between the mid-line and 

 Burdach's tract. Towards the end of the second month, the anterior white commissure is 

 indicated by the oblique transverse ingrowth of axones into the most ventral part of the floor- 

 olate as they make their way to the opposite side. Meanwhile the anterior median fissure has 



Fig. 906. 



, Posterior columr 



Fig. 907. 



Root-fibreJ 



Dorsal zone 



Ependymal 



layer 



Ventral zone 



GoU's tract 



Burdach's tract 



Posterior median septum 

 Root-fibres 



Dorsal zone 



Pia mater 



Anterior column 



Developing spinal cord of about seven and one-half 

 weeks. X 44. (His.) 



Ventral zone 



Anterior median fissure Anterior Root-fibres 



with pial process 



Developing spinal cord of about three months. X 30. {His.) 



become deeper and narrower in consequence of the increased bulk of medio-ventral parts of the 

 cord. As the fissure is thus differentiated the process of mesoblastic tissue, which from the 

 earliest suggestion of the groove occupies the depression, is correspondingly elongated and 

 affords a passage for the blood-vessels destined for the nutrition of the interior of the cord. 

 Ui>til the third month the gray matter, derived from the nuclear layer, is much more voluminous 

 than the surrounding marginal layer, which, so far as the contribution of ner\ous elements is 

 concerned, is passive, since its conversion into the white matter depends upon the ingrowth 

 of axones from the neurones situated either within or outside the cord. 



The development of the individuat fibre-tracts includes two stages, between the comple- 

 tion of which a considerable, and sometimes a K^ng, period intervenes. The first marks the 

 invasion of the supporting tissue of the marginal zone by the ingrowing a.xones as naked a.xis- 

 cylinders ; the second witnesses the clothing of these fibres with myelin. The period between 

 the appearance of the tract and the development of the medullary coat is variable. In some 

 cases, as in the great cerebro-spinal motor paths, although the fibres grow into the cord during 

 the fifth month of fcetal life, myelination does not begin until shortly before birth and is not 

 completed until after the second year. In other cases, as in the direct cerebellar, a period of 

 three months, from the third to the sixth, elapses. It is probable that the acquisition of the 

 medullary coat commences before the functional activity of the fibres begins, although, such 

 stimulation undoubtedly assists ; further myelination proceeds gradually along the course of 

 the fibius and in the direction of conduction. 



