ANATOMY AND PHYSIOLOGY OF NERVOUS SYSTEM. 629 



Internal Structure of the Medulla. The medulla oblongata, like 

 the spinal cord, is formed of nerve-cells, nerve-fibers, and a mesh- 

 work of neuroglia. As it is a continuation of the cord, one ought 

 to find the white columns and central axis common to the spinal 

 cord. As a matter of fact, the constituent elements of the cord are 

 found in the medulla, but their position is changed very much. The 

 cells forming the nuclei of nerves are analogous to those of the cord, 

 but are more isolated. They also give exit to fibrils which unite 

 them to other cells in the opposite half of the medulla and in the 

 brain proper, and to nerves of which they are the seat of origin. In 

 the medulla the grouping of these nuclei is quite different to that 

 found in the spinal cord. However, it is always the same central 

 gray substance, but modified in its form and arrangement. The 

 gray matter is cut here and there by white columns and their frag- 

 ments. 



To understand this new disposition of the gray matter it is 

 necessary to recall that at the level of the medulla the central gray 

 substance of the cord has been pushed backward by reason of sev- 

 eral factors. These are : the separation of the restif orm bodies, the 

 passage outward of the posterior columns, and the formation of the 

 rhomboid sinus. The latter is so arranged as to form the floor of 

 the fourth ventricle. The posterior horns have become separated 

 and are so rotated upon themselves as to be thrown outward and 

 thus placed at the external part of the fourth ventricle. The 

 anterior cornua have their bases placed upon the floor of the fourth 

 ventricle on each side of the median raphe. 



The isolated horn of gray matter is afterward known as the 

 nucleus lateralis. 



Further, the crossed pyramidal tracts of fibers are carried for- 

 ward, outward, and upward. By the oblique passage of these numer- 

 ous white fibers through the gray matter of the anterior horn the 

 anterior horn is broken up so that the caput is entirely separated 

 from the remainder of the gray matter. The fibers in passing 

 through the base of the anterior horns to decussate upon the median 

 line with those of the opposite side give rise to the reticulated for- 

 mation of Deiters and to the raphe of Stilling. 



FORMATIO RETICULARIS. The formatio reticularis is an asso- 

 ciated system of the short fibers with nerve-cells and is met with 

 at any point between the spinal cord and the optic thalamus. These 

 fibers run at right angles to one another. It is the result of the 

 decussation of the crossed pyramidal and arciform fibers which, in 



