CHAPTER XX. 



THE ANATOMIC RELATIONS OF THE MEDULLA OBLONGATA; 



THE ISTHMUS OF THE ENCEPHALON; THE CORPORA 



QUADRIGEMINA; THE BASAL GANGLIA. 



THE MEDULLA OBLONGATA. 



The medulla oblongata is that portion of the central nerve system im- 

 mediately superior to and continuous with the spinal cord. It has the shape 

 of a truncated cone, the base of which is directed upward, the truncated 

 apex downward. It is 38 mm. in length, 18 mm. in breadth, and 12 mm. 

 in thickness. By the continuation upward of the anterior and posterior 

 median fissures, the medulla is divided into symmetric halves (Figs. 238 and 

 239). Like the cord, of which it is a continuation, it is composed of white 

 matter externally and gray matter internally. 



Structure of the Gray Matter. The gray matter of the medulla is 

 continuous with that of the cord, though owing to the shifting of position of 

 the different tracts of the white matter it is arranged with much less regular- 

 ity. The appearance which the gray matter presents on transverse section 

 varies also at different levels. 



At the level of the first cervical nerve the posterior horns are narrow, 

 elongated, and directed outward. The lateral horns are well developed and 

 present a collection of cells near their bases which can be traced upward and 

 downward for some distance. At the level of the decussation of the py- 

 ramidal tracts the head of the anterior horn becomes detached from the rest of 

 the gray matter and is pushed backward toward the posterior horn; the bases 

 of the anterior horns become spread out to form a layer of gray matter near the 

 dorsal aspect of the medulla. Transverse sections of the medulla at all 

 levels show a more or less extensive network of nerve-fibers known as the 

 reticular formation. In its meshes are found collections of nerve-cells of 

 varying size. Toward the dorsal aspect of the medulla special groups 

 of cells are found from which axons arise to become the fibers of various 

 efferent cranial nerves, e.g., hypoglossal, efferent fibers of the vagus, and 

 glossopharyngeal. 



Structure of the White Matter. The white matter is composed of 

 nerve fibers supported by connective tissue and neuroglia. It is subdivided 

 on either side by grooves into three main columns: viz., an anterior column 

 or pyramid, a lateral column, and a posterior column. 



The anterior column or pyramid is composed partly of fibers continuous 

 with those of the anterior column of the spinal cord (the direct pyramidal 

 tract), and partly of fibers continuous with those of the lateral column of the 

 cord of the opposite side (the crossed pyramidal tract) , which decussate at 

 the anterior portion of the medulla. The united fibers can be traced up- 

 ward to the pons, where they disappear from view. 



