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 > i ts 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 hi 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, 



