250 THE ENGINES OF THE HUMAN BODY 



reached. Nerve fibres were laid down in the body in the 

 same continuous and systematic way as men learned to 

 lay telephone wires. There was no indication that nerves 

 were made up of independent units or cells. In the 

 brain, spinal cord, and in ganglia, however, units were 

 discovered of the most diverse kinds. Anatomists noticed 

 that these units or cells gave off branches. Some units 

 seemed to have the appearance of microscopic carrots, 

 with feathery processes or dendrites passing out from one 

 end and a long tapering root ending a fibre-like continua- 

 tion from the other. It was suspected at quite an early 

 stage of microscopic investigation that every nerve fibre 

 ended in a nerve cell, but that was not fully proved until 

 1885. In this year Professor His of Leipzig had finished 

 a long study of the changes which take place in the spinal 

 cords of developing human embryos. He found that 

 the brain and cord were at first made up entirely of units 

 or cells. He detected the roots- of spinal nerves in the 

 process of development. The fibres which make up an 

 anterior root began, he observed, as sprout-like out- 

 growths from a colony of units within the cord ; a nerve 

 sprout grew out from a cell or unit just as a tender shoot 

 does from a sown wheat grain. The sprouts spread out- 

 wards exactly in the direction which led them to the part 

 they were designed to supply. How they are guided we 

 do not yet know. At that early stage of development of 

 the body nerve-sprouts, even those which form the nerve 

 fibres to the muscles in the sole of the foot, have not far 

 to go. The fibres of the posterior root, Professor His 

 observed, were also formed as outgrowths from units or 

 cells. The colony which gave rise to them lay outside the 

 cord and ultimately became included in the swelling or 

 ganglion of the posterior root. Almost at their appear- 

 ance the outgrowths of the hinder cells divided into two 

 branches : one passed into the substance of the spinal cord 

 and crept up towards the medulla, ultimately establishing 

 many connections with elements of the cord (see fig. 47B) ; 

 the other division grew outwards and, in company with 

 the corresponding anterior root fibres, reached its right 



