288 . THE NERVOUS SYSTEM 



its likeness to a branching tree, u the tree of life." The 

 gray surface is arranged in parallel ridges, or laminated 

 folds, differing from the irregular convolutions of the 

 cerebrum . 



The pons Varolii also contains white and gray matter. 

 In it white fibers pass upward to connect the medulla ob- 

 longata with other parts of the brain. It contains, also, 

 the strands of white matter which form the middle 

 peduncle of the cerebellum. The pons is, therefore, as its 

 name implies, the passage, or bridge, by means of which 

 connection is made between all parts of the nervous 

 system. 



425. The Medulla Oblongata, or Spinal Bulb, is the enlarged 

 upper portion of the spinal cord contained within the 

 cavity of the cranium (Figs. 129, 130, and 134). It is 

 conical in shape, about one inch in thickness at its broad- 

 est part, and about one inch arid a quarter in length, and 

 consists of two symmetrical halves. On the forward, or 

 anterior, surface is seen a deep groove, which is a con- 

 tinuation of the anterior fissure of the cord. A similar 

 but more shallow fissure forms the posterior division 

 between the halves. Just as the cord begins to expand 

 into the medulla, many of the fibers which compose the 

 lateral columns of the cord cross from one side to the 

 other. These bundles of white fibers are called, in 

 the medulla, the pyramids, and their crossing is called 

 the decussation of the pyramids. Other fibers of the 

 pyramidal tracts cross lower down at different levels in 

 the cord. In general, a rearrangement of the fibers of the 

 cord takes place in the medulla: What it is especially 

 important to notice is that each tract of fibers in the cord 

 has connection through the medulla with the centers in 

 both the cerebellum and the cerebrum. 



