NERVOUS SYSTEM. 139 
while those of roof and floor plates do not. As a result the sides soon 
extend downward on either side beyond the floor plate, thus forming a 
longitudinal groove, the anterior or ventral fissure of the cord, ex- 
tending its whole length (fig. 144, B). The roof plate, on the other 
hand, is at first carried upward by the growth, thus increasing the ver- 
tical diameter of the central canal. Then the dorsal portion of the 
tube closes up—the exact steps are uncertain—and later the tissue 
along the line of closure is invaded by connective tissue and 
blood-vessels, the result being the dorsal or posterior fissure of the 
cord. 
Besides the increase in the number of cells, the sides of the cord are 
modified in other ways. Those cells which line the cavity—floor, roof 
and sides—retain their epithelial character, never develop nervous struc- 
tures, and are known as the ependyma. ‘The remaining cells become 
differentiated in two directions. Some develop processes which sur- 
round and support the others, these forming the neuroglia (‘glia’), 
while the others form the true nervous tissue—ganglion or nerve cells. 
In the primitive condition the primitive nerve cells have no connexion 
with distant points and hence cannot function. These connexions are 
established by protoplasmic outgrowths from each cell, these forming 
the fibres (dendrites or axons). Some of these extend directly out- 
ward from the cord as nerves (see below), but others run for a greater 
or less distance on the external surface of the cord, and since these 
have medullary sheaths (p. 20) and are consequently white, these 
tracts constitute the white matter of the cord, in contrast to the gray 
matter formed by the cell bodies and neuroglia. 
In sections of the adult cord the gray matter has something of the 
shape of the letter H,, its uprights forming the anterior and posterior 
horns or cornua, while the cross-bar extends above and below the 
central canal, from one side to the other. Physiological phenomena 
and matters of nerve origin lead to the recognition of a lateral cornu 
on either side, in the lateral prominence of gray matter. Since both 
dorsal and ventral cornua approach the surface of the cord to connect 
with the nerve roots described below, they divide the white matter into 
three tracts on either side, known as the anterior, lateral and pos- 
terior columns of the cord, each subdivided in the higher vertebrates 
into several bundles. As this white matter is composed of nerve fibres, 
it follows that these columns are the tracts by which nervous impulses 
are carried to and from the brain, the anterior columns leading from, 
