SPINAL COED AND NEEVES 



285 



descending in the dorsal column and the region round the dorsnl 

 horn (Fig. 172). Both these branches give off collaterals at fairly 

 close intervals, which run towards the grey matter, penetrate it, 

 and ramify around its cells. Some of these collaterals run to the 

 cells of the ventral horn on the same side ; others enter into 

 direct relation with the cells of 

 Clarke's column or the solitary 

 cells of the dorsal horn. 



To summarise the facts most 

 essential to physiology : The fibres 

 of the peripheral nerves which 

 emerge by the ventral roots have 

 their cells of origin in the ventral 

 horn of the cord, and those which 

 enter by the dorsal roots originate 

 not from the cells of the cord, but 

 from the spinal ganglia. In the 

 grey matter of the cord the nerve- 

 tibres of the two roots are closely 

 related, so that transmission of the 

 excitations from one root to the 

 other is possible, either by simple 

 contact between the ends of the 

 neurones, or by the anastomosis of 

 the neurones among themselves 

 into a common fibrillary network. 

 Lastly, the nerve-fibres which make 

 up the dorsal white column of the 

 cord are the prolongation of the 

 peripheral fibres that enter by 

 the dorsal roots ; but the fibres 

 that constitute the ventro-lateral 

 columns are independent of the 

 peripheral neurones. 



Investigations on the embryo- 

 logical development of the spinal 

 cord, pathological observations on 

 spinal diseases in man, and the 

 effects of partial sections of the 

 cord in animals, have yielded the 

 highly important result that the columns which make up the 

 white matter of the cord are not uniform masses of nerve-fibres, but 

 can be subdivided into well-differentiated bundles or tracts. The 

 enibryological investigations of Flechsig led to the very important 

 conclusion that the development of the myelin sheath does not 

 take place simultaneously on all the longitudinal fibres of the 

 spinal cord, but it occurs earlier in certain bundles or tracts of 



FIG. 172. Longitudinal section of dorsal 

 column of spinal cord of chick on eighth 

 day of incubation. (Ramon y Cajal.) 

 Shows the course of five entering fibres 

 of dorsal root, and some longitudinal 

 fibres of ventral column. A, A, fibres of 

 dorsal root; B, bifurcation of one in 

 form of Y ; C, D, origin of collateral 

 branches ; B, fibres of Qoll's tract, also 

 giving oft' collaterals. 



