George L. Streeter 25 



over, was a " Y " form seen, where the entering fibre had both a 

 eephalad and caudad collateral. That so few fibres take a downward 

 course accounts for the fact that in the 35th and 36th segments, where 

 there is a pronounced increase in the size of the dorsal nerve roots, the 

 corresponding increase in the size of the dorsal funiculi is in the cephalic 

 direction. Furthermore, the course of these fibres in the dorsal funiculi 

 cannot be a long one, and this is shown by the rapid decrease in the 

 size of the funiculi coincident with the decrease in the number of enter- 

 ing fibres, a fact which we have already referred to in the consideration 

 of the diagram, Fig. 4. 



The collaterals from the dorsal funiculi to the grey matter vary in 

 number in correspondence to the number of fibres from the dorsal 

 nerve roots. In the lumbo-sacral enlargement these fibres enter the 

 dorsal horn as a large, strand of fibres which could be traced to the 

 region at the base of the horn. In the cervical region fibres entering 

 the grey matter are found only as single separate collaterals. 



No definite subdivision of these funiculi into separate fasciculi or 

 tracts could be made. In general, however, the fibres of the ventral 

 one-third are smaller and form a triangular field of fine fibres, aver- 

 aging .2 fi. These are apparently association fibres. This field is not 

 present in the lumbo-sacral enlargement; here the grey commissure is 

 absent, and the dorsal funiculi are separated by the sinus rhomboideus 

 and lie further dorsal. The size of the fibres of this enlargement is 

 uniformly large, the myeline ring in Weigert preparations averaging 

 1.0 to 1.5 fjb. We have already seen that the majority of the fibres of 

 this region do not remain in the dorsal funiculi for a course of more 

 than 3 to 4 segments, and that a small proportion of them reach the 

 medulla through this tract. It would seem, then, tliat large fibres do 

 not necessarily indicate long fibres; because in the lumbo-sacral en- 

 largement the fibres are uniformly large and it is right here that we 

 have shown that at least three-fourths of the fibres have a course in the 

 dorsal funiculi shorter than 4 segments. 



The lateral funiculi present an inner zone of fine fibres and an 

 outer zone of coarser fibres, the latter fibres averaging 1.0 yu,. The inner 

 zone, or formatio reticularis, makes up a third to one-half the area. It 

 is connected with the grey substance by numerous radiating strands of 

 fibres, and apparently consists of association bundles. The outer field 

 is connected with the central grey substance by less numerous strands 

 of fibres. It is in this outer zone that Friedldnder^^ found ascending 



^^ Friedlander, L. c. 



