THE NERVOUS SYSTEM OF VERTEBRATES 



299 



finally the cell 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 exten- 



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 ; b, c, d, motor iierv,e 

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



, sion of the 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 form& 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 



