WHITE MATTER OF THE CORD. 351 



tactile impressions may reach the cerebrum without passing through 

 the gray substance in the spinal cord; but it does not exclude the 

 possibility of a part of the impulses of the muscular and tactile 

 senses being transferred from a lower to a higher neurone in the 

 spinal gray substance. 



2. WHITE MATTER OF THE CORD. 



The white matter (Fig. 101) of the spinal cord (substantia alba 

 spinalis} is disposed in its peripheral area and in the white ante- 

 rior commissure. It is composed of medullated nerve fibers 

 (axones and collaterals) imbedded in a small amount of neuroglia, 

 and supported by a connective tissue network derived from the 

 pia mater. Like the gray matter, it is richly supplied with blood- 

 vessels. The fibers of the spinal cord run transversely, dorso- 

 ventrally and longitudinally. 



The transverse fibers, which are usually somewhat oblique 

 in direction, comprise (i) those running from the longitudinal 

 tracts into the gray matter or out of the gray matter into such 

 tracts; (2) the axones of intrinsic neurones which connect the two 

 crescents at nearly the same level; and (3) posterior root-fibers 

 running through the posterior commissure to the opposite crescent. 



The white anterior commissure of the spinal cord (commissura 

 anterior alba} is the only definite lamina of transverse fibers in 

 the cord (Fig. 101). It connects the anterior and lateral white 

 columns of the cord with the opposite gray crescent and the two 

 crescents with each other. It is located in front of the gray an- 

 terior commissure, forming the floor of the anterior median fissure. 

 It is composed of medullated fibers belonging to (a) the anterior 

 pyramidal tract, (b) the anterior fasciculus proprius, (c) the as- 

 cending anterior cerebello-spinal and spino^thalamic tracts; (d) it 

 comprises the crossed fibers to the anterior roots of the spinal 

 nerves, and (e) the decussating dendrites between the anterior 

 columnae. 



The dorso-ventral fibers of the spinal cord (Fig. 104) are 

 (a) those of the anterior roots of the spinal nerves, in their course 

 from the gray matter to the surface of the cord; (b) those of the 



