Hardest Y, Spinal Cord of the Emit. 93 



along the pia collect rapidly in approaching from both direc- 

 tions to form the nerve roots of a segment, so must the axones 

 continued from them within the substance of the cord increase 

 in abundance toward the level at which the nerve roots are 

 formed and thus, together with their divisions and collaterals, 

 must they contribute to the segmental enlargement near this 

 level. As Streeter points out for the ostrich, the emu with 

 its large spinal cord must be an animal, the principal nerve con- 

 trol of whose body is supplied from its spinal cord segments 

 simply — "an animal that works chiefly with its primary appara- 

 tus." From the fact that, with the exception of the slender 

 caudal extremity, the most slender region of the cord is in the 

 neck, the great mass of its dorsal root axones must be of short 

 course. Indicated by the segmental swellings, it is very prob- 

 able that a large proportion of the dorsal root fibers of a seg- 

 ment are distributed in that segment alone, making many of the 

 reactions of the simple reflex character. Accordingly, the de- 

 scending cerebro-spinal fasciculi would be small, as must be the 

 case from the proportions of the cord. 



As the efferent or ventral root fibers are absent along the 

 surface of the cord midway between the nerve pairs and begin 

 to emerge and collect rapidly toward the level at which the 

 root is formed, so the cell-bodies giving origin to these fibers 

 show a tendency to cluster toward the middle of the segment. 

 However, this clustering is not so great as might be expected. 

 The ventral columns maintain their shape and contain large 

 cells throughout the segment. For three of the longer seg- 

 ments counts were made of the cells contained in sections 

 through the segmental enlargements and in sections from mid- 

 way between adjacent nerve pairs. Each count involved 20 

 transverse sections of 30 ^ thickness and included all cell-bodies 

 having a mean diameter of 10 // and above. The following 

 gives the average number of cells per section found in each of 

 the two localities of these segments: 



