THE NERVOUS SYSTEM OF VERTEBRATES 299 



becomes uni-polar, giving off one process which divides by a T-shaped 

 junction into two, one of which runs towards the spinal cord, while the other 

 takes a peripheral course as the afferent nerve fibre. The central nervous 

 system thus becomes provided with a ' way in ' and a ' way out ' for the chain 

 of impulses concerned in a nervous reaction or reflex action. The further 

 development of the spinal cord is mainly determined by the extension of the 



FIG. 142. Section through developing spinal cord and nerve-roots from chick 



embryo of fifth day. (CAJAL.) 



A, ventral root ; B, dorsal root ; c, motor nerve-cells ; D, sympathetic ganglion- 

 cells ; E, spinal ganglion cells still bi-polar ; F, mixed nerve ; 6, c, d, motor nerve 

 fibres to /, developing spinal muscles ; e, a sensory nerve-trunk. 



axons of the cells outside the tube of cells themselves, and by the provision 

 of the ' long paths ' which are a necessary condition of increased efficiency of 

 the reacting organ. Some time after the outgrowth of the axon a medullary 

 sheath is formed, apparently by the agency of the axon itself, so that each 

 group of axons leaving or entering the cord forms a bundle of medullated 

 nerve fibres. The long branches of the posterior or dorsal roots running up 

 towards the head form a mass of fibres behind the tube of cells known as 

 the posterior columns. Fibres starting in the spinal cord itself run upwards 

 and downwards to end in other parts of the cord, or in the more anterior 

 divisions of the central nervous system forming the brain, and surround the 

 neural tube on its ventral and lateral aspects with a sheath of white matter. 

 To these white fibres are added others, which take origin in the brain and pass 

 all the way down the cord. Meanwhile the cells themselves become separated 



