STRUCTURE OF THE SPINAL CORD 



319 



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 columris. 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 composed. 



For tracing out the course of the different axon systems in the central nervous 

 system several methods arc available. 



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

 with all its processes, namely, the intravitum 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 within the nerve cells and their 

 processes ; but for some unexplained reason the impregnation is not general, and is 

 confined to a small percentage of the neurons. If 

 the precipitate were diffuse, even a thin section 

 would be absolutely opaque ; since it is partial, 

 thick sections may be cut and, after clearing, allow 

 the tracing of the processes of the few impregnated 

 nerve cells through the whole thickness of the section. 

 We may in this way get sections 0-1 mm. thick at 

 the point of entrance of a posterior nerve root, and 

 trace out the course and ending of a large number of 

 the fibres composing the nerve root, or we may in a r/fc - 

 section involving the anterior nerve root trace the 

 course of an axon of an anterior cornual cell out of 

 the cord into the root. This method is of no use in 

 tracing any given nerve fibre through the whole 

 length of the cord. For this purpose however 

 several methods are available. 



(6) MYELINATION METHOD OF FLECHSIG. 



Nerve fibres at their first formation as axons of a T 



, , , jll FIG. 156. Section throusrh the cer- 



nerve cell are non-medullated, the medullary sheath vical 8pinal cord of a new . born 



being formed later with the beginning of function of child, stained by -Weigert's 



the nerve. It has been shown by Flechsig that the 



myelination does not occur simultaneously through 



all parts of the central nervous system, but that it 



is later in proportion as the nerve fibre is more 



recent in the phylogenetic history of the animal. root zon e ; 



The cord in its most primitive form can be regarded fibres. 



as a series of ganglia presiding over the different 



segments of the body. The most primitive fibres therefore would be those which run 



from the periphery of the body to each segment and from each segment out to the 



muscles, and so a medullary sheath is first formed in a number of the fibres entering 



and leaving the cord in the nerve-roots. Next in order of myelination are those 



method, to show absence of 

 medullation in pyramidal tract. 



ca > anterior commissure ; Fp, 



rp', posterior 

 (BECHTEREW.) 



root 



