746 



A MANUAL OF PHYSIOLOGY 





spinal artery, and cause softening mainly of the grey matter of the 

 lumbar portion of the cord. When the abdominal aorta of a rabbit 

 is temporarily compressed (for about an hour) below the origin of 

 the renal arteries, the grey matter of the corresponding portion 

 of the cord is so seriously injured that it and the fibres that arise 

 from it degenerate, while the fibres whose cells of origin are not 

 situated in this part of the grey matter are not affected, or at least 

 completely recover. 



Certain tracts may also be marked out by means of the electrical 



variation, which gives token 

 of the passage of nervous im- 

 pulses along them when por- 

 tions of the central nervous 

 system or peripheral nerves 

 are stimulated (Horsley and 

 Gotch). 



Development of the Central 

 Nervous System. Very early 

 in development (Fig. 297) the 

 keel of the vertebrate embryo 

 is laid down as a groove or 

 gutter in the ectoderm of 

 the blastodermic area (Chap. 

 XIV.). The walls of this ' me- 

 dullary ' or ' neural ' groove 

 grow inwards, and at length there is formed, by their coalescence, 

 the 'neural canal ' (Fig. 298), which expands at its anterior end to 

 form four cerebral vesicles (Fig. 299). Thus there is a continuous 

 tunnel from end to end of the primary cerebro-spinal axis ; and this 

 persists as the central canal of the spinal cord and the ventricles of 





FIG. 297. FORMATION OF THE NEURAL 

 CANAL AT AN EARLY STAGE (AFTER 

 BEARD). 



FIG. 298. NEURAL CANAL AT A 

 LATER STAGE (AFTER BEARD). 



the brain, whose ciliated epithe- 

 lium represents the ectodermic 

 lining of the primitive neural canal. 

 In the adult portions of the canal 

 may become obliterated from an 

 overgrowth of the lining cells, and 

 the cilia are, if present at all, less 

 distinct than in the child, and far 

 less distinct than in the lower animals. From the wall of this canal 

 is formed the cerebro-spinal axis, in which developing nerve-cells 

 or neuroblasts soon become differentiated from the supporting cells 

 or spongioblasts, and wander outwards from the neighbourhood of 

 the central canal (Fig. 309) till their further progress is checked by 

 the barrier of the marginal veil, a closely- woven network or thicket, 

 into which the processes of the spongioblasts break up at the out- 

 side of the primitive cerebro-spinal axis. Although the neuroblasts 



