CHAPTER XL 



ARRANGEMENT OF STRUCTURAL ELEMENTS. 



Arrangement of spinal nerve roots Meynert's scheme Finer 

 anatomy Size of nerve elements As related to size of 

 animal Mass of the cell Mass of the nucleus Innervation 

 of the frog's leg Gaule's observations on numerical relations 

 Connection of nerve elements with one another Course of 

 nerve impulse Complexity of central system Variations in 

 complexity. 



A CROSS-SECTION of the spinal cord like that given in 

 Fig. 40 will show the arrangement of the afferent and 

 efferent elements. 



Such a section gives the structure of a modified 

 segment of the cord, and since the entire central 

 system is regarded as composed of such segments, more 

 or less modified at the cephalic end, this structure may 

 be regarded as typical. The afferent fibres arrive at the 

 centre by way of the dorsal roots, and the efferent fibres 

 leave it mainly by way of the ventral roots, but in small 

 measure by the dorsal roots also. Toward the cephalic 

 end of the cord and in the bulb the fibres carrying the 

 efferent impulses, but which do not pass out by the 

 ventral roots, become separated and form a lateral root 

 intermediate between the other two. The central cells 

 not shown in the figure are located in both the dorsal 

 and the ventral zones of the developed cord. These 



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