574 



DEVELOPMENT OF THE CEREBRO-SPINAL AXIS. 



The SPINAL CORD has been found by Kblliker already in the form of a cylinder in 

 the cervical region of an embryo four weeks old. Ununited borders have been seen 

 by Tiedemann in the ninth week towards the lower end of the cord, the perfect closing 

 of the furrow being delayed in that part, which is slightly enlarged and presents a 

 longitudinal median slit, analogous to the rhomboidal sinus in birds. 



The anterior fissure of the cord is developed very early, and contains even at first a 

 process of the pia mater. 



The cervical and lumbar enlargements opposite the attachments of the brachial and 

 crural nerves, appear at the end of the third month : in these situations the central 

 canal, at that time not filled up, is somewhat larger than elsewhere. 



Fig. 388. 



Fig. 388. TRANSVERSE SECTION OF HALF 

 THE CARTILAGINOUS VERTEBRAL COLUMN 

 AND THE SPINAL CORD IN THE CERVICAL 

 PART OF A HUMAN EMBRYO OF FROM 

 NINE TO TEN WEEKS ^from Kolliker). J^ 



c, central canal lined with epithelium ; 

 a, anterior column; p, posterior column; 

 p', band of Goll ; g, ganglion of the 

 posterior root ; p r, posterior root ; a r, 

 anterior root passing over the ganglion ; 

 dm, dura matral sheath, omitted near p r, 

 to show the posterior roots ; b, body of 

 the vertebra ; c h, chorda dorsalis ; n a, 

 neural arch of the vertebra. 



At first the cord occupies the whole 

 length of the vertebral canal, so that 

 there is no cauda equina. In the fourth 

 month the vertebrae begin to grow more 

 rapidly than the cord, and the latter 

 seems as it were to have retired up into 

 the canal, and the elongation of the roots 

 of the nerves which gives rise to the cauda 

 equina is commenced. At the ninth 

 month, the lower end of the cord is 

 opposite the third lumbar vertebra. 



In textual composition the spinal cord consists at first, after the completion of its 

 cylindrical form, entirely of uniform-looking cells. These separate into two layers, 

 the inner of which forms the epithelium and surrounding connective tissue, or neu- 

 roglia of the central canal, while the outer forms the grey substance of the cord. The 

 white substance appears later than the grey, forming a layer external to it, and sepa- 

 rated from an early period into an antero-lateral and a posterior column on each side. 

 At a somewhat later period the white mass of these columns, increasing greatly in 

 size, gives rise to the formation and gradually increasing depth of the anterior and 

 posterior median fissures. At the same time, however, the lateral masses of grey 

 substance also undergo extension in the parts named the cornua. It would appear 

 that the integral nerve-fibres are at first developed from radiating processes which 

 proceed from the cells of the grey substance. (Kolliker, Entwicklungsgeschichte ; 

 Lockhart Clarke, in the Phil. Trans. 1862 ; Bidder und Kupfer, Untersuch. iib. d. 

 Euckenmark, Leipz., 1857.) 



It may also be mentioned that, according to Remak and Kb'lliker, the roots of the 

 spinal nerves and the ganglion are at first unconnected with the cord. The mass of 

 blastema forming the ganglion first becomes apparent, and from this the posterior root 

 seems to gro w towards, and later to become attached to, the cord ; while the anterior roots 

 seem to extend outwards from the cord and to unite themselves later with the nerve. 

 The central canal is at first nearly cylindrical ; it then becomes flattened at the 

 sides, projecting deeply backwards and forwards. Between the eighth and tenth 

 weeks it is greatly narrowed, and subsequently, being more and more confined to 

 the centre, it ultimately diminishes to a small tube. The epithelial cells which line 

 it from the first are long or columnar, and they retain this form in the adult. 



