360 



PHYSIOLOGY 



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 fibers composing the 

 nerve-root, or we may in a 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 a.vailable. 



(b) MYELINATION METHOD OF 

 FLECHSIG. Nerve fibres at their first 

 formation as axons of a nerve-cell 

 are non-medullated, the medullary 

 sheath being formed later with the 

 beginning of function of the nerve. 

 It has been shown by Flechsig that 

 the myelination does not occur simul- 

 taneously 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. The cord in its 

 most primitive form can be regarded 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 myelina- 

 tion are those fibres which connect different segments of the cord, 

 the internuncial or intra-spinal fibres. Next come those fibres 

 which connect the spinal cord with the cerebellum. Last of all 

 to receive a medullary sheath are the fibres which take a direct 

 course from the cerebral cortex to the spinal cord. These are called 

 the pyramidal tracts, and in man are not medullated until the first 

 month after birth (Fig. 157). 



FIG. 157. Section through the cer- 

 vical spinal cord of a new-born 

 child, stained by Weigert's method, 

 to show absence of medullation 

 in pyramidal tract, 

 car., anterior commissure ; Fp, 

 crossed pyramidal tract ; Fe, direct 

 cerebellar tract ; Zrp, posterior root 

 zone ; rp', posterior root-fibres. 

 (BECHTEREW.) 



