THE CENTRAL NERVOUS SYSTEM 



659 



arteries, stick in the branches of the anterior 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 

 impulses along them when por- 

 tions of the central nervous 

 system or peripheral nerves are 

 stimulated (HorsleyandGotch.) 



Development of the Central 

 Nervous System. Very early 

 in development (Fig. 224) the 

 keel of the vertebrate embryo is laid down as a groove or gutter in 

 the epiblast of the blastodermic area (Chap. XIV.). The walls of 

 this * medullary groove ' grow inwards, and at length there is formed, 

 by their coalescence, the 'neural canal' (Fig. 225), which expands at 

 its anterior end to form four cerebral vesicles (Fig. 226). Thus 

 there is a continuous tunnel from end to end of the primary cerebro- 





FIG. 224. FORMATION OF THE NEURAL 

 CANAL AT AN EARLY STAGE (AFTER 

 BEARD). 



spinal axis ; and this persists as the 

 central canal of the spinal cord and 

 the ventricles of the brain, whose 

 ciliated epithelium represents the 



FIG. 225. NEURAL CANAL AT A epiblastic lining of the primitive 

 LATER STAGE (AFTER BEARD), neural canal. In the adult portions 



of the canal may become obliter- 

 ated from an evergrowth 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 neuro- 

 blasts soon become differentiated from the supporting cells or 

 spongioblasts, and wander outwards from the neighbourhood of the 

 central canal (Fig. 233) till their further progress is checked by the 

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



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