OF THE NERVOUS SYSTEM. 381 



occupying the two lateral halves of the cord, are united with each 

 other by a transverse band of the same substance, which is called 

 the gray commissure of the cord. Directly in front of this is a trans- 

 verse band of white substance, connecting in a similar manner the 

 white portions of the two lateral halves. It is called the white 

 commissure of the cord. 



The spinal nerves originate from the cord on each side by two 

 distinct roots ; one anterior, and one posterior. The anterior root 

 (Fig. 127, d) arises from the surface of the cord near the extremity 

 of the anterior peak of gray matter. The posterior root (e) origi- 

 nates at the point corresponding with the posterior peak of gray 

 matter. Both roots are composed of a considerable number of 

 ultimate nervous filaments, united with each other in parallel 

 bundles. The posterior root is distinguished by the presence of a 

 small ganglion (c), which appears to be incorporated with it, and 

 through which its fibres pass. There is no such ganglion on the 

 anterior root. The two roots unite with each other shortly after 

 leaving the cavity of the spinal canal, and mingle their filaments 

 in a single trunk. 



It will be seen, on referring to the diagram (Fig. 127), that each 

 lateral half of the spinal cord is divided into two portions, an 

 anterior and a posterior portion. The posterior peak of gray mat- 

 ter comes quite up to the surface of the cord, and it is just at this 

 point (e) that the posterior roots of the nerves have their origin. 

 The whole of the white substance included between this point and 

 the posterior median fissure is called the posterior column of the 

 cord. That which is included between the same point and the 

 anterior median fissure is the anterior column of the cord. The 

 white substance of the cord may then be regarded as consisting 

 for the most part of four longitudinal bundles of nervous filaments, 

 viz., the right and left anterior, and the right and left posterior 

 columns. The posterior median fissure penetrates deeply into the 

 substance of the cord, quite down to the gray matter, so that the 

 posterior columns appear entirely separated from each other in a 

 transverse section ; while the anterior median fissure is more shal- 

 low and stops short of the gray matter, so that the anterior columns 

 are connected with each other by the white commissure above men- 

 tioned. 



By the encephalon we mean the whole of that portion of the 

 cerebro-spinal system which is contained in the cranial cavity. It 

 is divided into three principal parts, viz., the cerebrum, cerebellum, 



