STRUCTURE OF THE SPINAL CORD 359 



(1) THE MOTOR CELLS, the largest of all, which send their axons 

 into the anterior roots, where they run to supply skeletal muscle fibres. 

 As a sub-group of these cells we may class the somewhat smaller cells 

 of the lateral horn, which in all probability send their axons by the 

 anterior roots to supply visceral muscles. Their axons can be dis- 

 tinguished from the motor axons by the smaller diameter of the nerve 

 fibres they form. They pass later from the mixed nerve along a 

 white ramus communicans into the sympathetic system, in the ganglia 

 of which "they end. 



(2) CELLS OF THE COLUMNS. As typical of these cells we may 

 take those which form Clarke's column. Their axons do not leave 

 the central nervous system, but pass out into the white matter to some 

 other part of the central nervous system, contributing thus to form the 

 white columns of the cord. 



(3) COMMISSURAL CELLS. These cells send their axon across 

 the middle line to the opposite side of the cord, making up a great part 

 of the anterior white commissure. 



(4) CELLS OF GOLGI. These cells are found chiefly in the posterior 

 horn. They are multipolar and are distinguished from all the other 

 cells by the fact that their axon does not pass far from the cell, but 

 rapidly breaks up into a number of branches which terminate in the 

 near neighbourhood of the cell giving off the axon. They may be 

 regarded as association cells, i.e. as serving to establish a functional 

 connection between many different cells at any given level of the grey 

 matter. 



The white matter of the cord is divided by the fissures already 

 described into anterior, lateral, and posterior columns. The nerve 

 fibres of which it is composed are all of them axons of nerve-cells 

 situated at different levels of the central nervous system or outside 

 the cord. Since the whole object of the study of the anatomy of 

 the cord is the tracing out of the systems of neurons of which it is 

 made up, and therefore of the possible paths of any reflexes or nerve 

 impulses through the cord, a mere anatomical differentiation of 

 different columns is quite useless unless we can determine in each 

 column the origin and destination of the fibres of which it is com- 

 posed. For tracing out the course of the different axon systems in 

 the central nervous system several methods are available. 



(a) HISTOLOGICAL. Two methods may be employed for staining 

 a nerve-cell with all its processes, namely, the intravitam staining 

 with methylene blue and the impregnation method invented by 

 Golgi. In the latter method, of which there are many modifications, 

 the nervous tissue is hardened in some chromate or bichromate, 

 and is then soaked in a solution of silver nitrate or mercuric chloride. 

 In this way a precipitate of silver or mercuric chromate is formed 



