THE CENTRAL NERVOUS SYSTEM. 253 



fibres. It is also a nerve-centre of complex constitution, in which 

 neurons terminate in teleneurites or arise in teledendrites. 



Some of the neurons within the cord are confined to its substance, 

 and constitute nervous connections between the different parts at 

 various levels. These may be termed longitudinal commissural 

 neurons, or association-fibres. Portions of such neurons are repre- 

 sented in the diagram of a cross-section of the cord (Fig. 236), 

 which also contains representations of some of the neurites in the 

 posterior spinal nerve-roots, with their collaterals ending in tele- 

 neurites within the gray matter (d). On the right side of the figure, 

 the nerve-cells, with their dendrites and the beginning of the neu- 

 rites, are shown. On the left side the neurites connected with cells 

 at another level are shown, re-entering the gray matter, where they 

 terminate in teleneurites. In studying this figure it must be borne 

 in mind that the teledendrites of the neurons on the right are in 

 close relations with the teleneurites of other neurons, and that the 

 teleneurites represented on the left are in close relations with the 

 teledendrites of other neurons. These association-neurons are, 

 therefore, merely links in chains of communicating neurons. They 

 are again represented in Fig. 239, I) and E. 



Aside from these association-neurites, the gray matter of the 

 cord receives innumerable collaterals from the neurites forming the 

 axis-cylinders of the nerves in the various columns of the white 

 matter. These collaterals terminate in teleneurites, which are in 

 close relations with the teledendrites of the neurons arising in the 

 cord. The distribution of these collaterals is represented in Fig. 

 237. The collaterals from the anterior column enter the anterior 

 horn of the gray matter, where they are chiefly distributed about 

 the large ganglion-cells in the antero-lateral portion of its substance 

 (Fig. 233, b; Fig. 236,,/), but may also extend to other parts of 

 the gray matter. The collaterals from the fibres in the lateral 

 columns of the white matter are most numerous near the pos- 

 terior horn, which they enter, many of them passing through the 

 gray matter behind the central canal and forming a part of the 

 posterior or gray commissure of the cord (Fig. 237, I). The col- 

 laterals from the posterior column are divisible into four groups : 

 first, those which are given off in the lateral portion of that column 

 (Fig. 237, G), and are distributed in the outer portion of the pos- 

 terior horn and in the substance of Rolando (Fig. 237, I) ; second, 

 those which end in Clarke's column (Fig. 237, J) ; third, those 



