336 PHYSIOLOGY 



towards the periphery of the body, where it ends in close connection 

 with the surface in the various sense-organs of the skin and muscles. 

 These collections of bi-polar cells form the posterior root ganglia. 

 In fishes they retain their primitive character throughout life, but in 

 mammals the bi-polar cell is to be found only in the spiral and vesti- 

 bular ganglia which give origin to the fibres of the eighth nerve. In all 



FIG. 143. Section through developing spinal cord and nerve-roots from 



chick embryo of fifth day. (CAJAL.) 



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

 ganglion- cells ; B, spinal ganglion- cells still bi-polar ; F, mixed nerve ; 

 &, c, d, motor nerve fibres to /, developing spinal muscles ; e, a sensory 

 nerve -trunk. 



the other ganglia the shape of the cell becomes modified by an approxi- 

 mation of the points of attachment of the two processes until 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 extension of the axons of the cells outside 

 the tube of cells themselves, and by the provision of the ' long 



