THE SPINAL CORD 



The spinal cord is made up not only of the neurons thus far described, whose cells lie in 

 it, and of the neuraxons which come down from cells in the brain, but also of a large number 

 of neuraxons which enter it from the posterior spinal ganglia. The researches of His ' have 

 shown that in these ganglia there develop cells which send out two processes. Plate XI. shows 

 such a ganglion from a chick embryo, containing a number of cells, each with two processes. One 

 process proceeds outward in the posterior spinal nerve, and passes to the skin or to some 

 internal organ. The other process proceeds inward in the posterior spinal nerve root and 

 enters the spinal cord. The latter process is usually smaller in its calibre than the former 

 one, as proven by Retzius. 2 In a later stage of development the body of the cell appears 

 to be pressed to one side, so as to lie not directly in the line of the branches, and then 

 the two branches become fused together, so that in an adult the appearance of the cell is 

 changed, and it looks like a pear with the stem divided and passing in two opposite 

 directions. There has been some discussion as to which of the two branches of these 

 cells is to be called a dendrite and which a neuraxon. As a matter of fact, no distinctive 

 differences are to be seen in the branches, and it is better to regard this type of cell as 

 different from the spinal neurons, and to maintain that these cells have two neuraxons and 

 no dendrites, especially as both branches receive a medullary sheath on leaving the cell. 

 It is known that sensations are conveyed from the skin to the spinal cord along these 

 branches, and it is evident, therefore, that the distal branch conveys the impulse to the cell, 

 and the proximal branch conveys it from the cell. It is not impossible that the cell has 

 little to do with the function of sensation, and that its only function is to maintain the 

 nutrition of its processes. It is certainly true that if either process is separated from its 

 cell it atrophies in its entire length. 



1 W. His, Arch. f. Anat. u. Phys. Anatom. Abtheil., 1887. 2 Retzius, Biol. Untersuchungen, Stockholm, I. 1890. 



