CHAPTER XVIII. 



THE MEDULLA OBLONGATA; THE ISTHMUS OF THE 

 ENCEPHALON; THE BASAL GANGLIA. 



r THE MEDULLA OBLONGATA. 



The .medulla oblongata is that portion of the central nerve 

 system immediately 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, 

 1 8 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. 211 and 212). 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 regularity. 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 forward and backward for some distance. At the 

 level of the decussation of the pyramidal tracts the head of the 

 anterior horn becomes completely 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 sec- 

 tions 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., 

 the hypoglossal, the efferent fibers of the vagus, and glosso-pharyngeal. 



Structure of the White Matter. The white matter is com- 

 posed 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. 



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