CHAPTER XXII 



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 ventral and dorsal 

 median fissures, the medulla is divided into symmetric halves (Figs. 232 and 

 233). 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 dorsal 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 ventral horn becomes detached from the rest 

 of the gray matter and is pushed backward toward the dorsal horn; the 

 bases of the ventral horns become spread out to form a layer of gray matter 

 near the dorsal aspect of the medulla. Transverse sections of the me- 

 dulla 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 funiculi: viz., an anterior 

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



The ventral funiculus or pyramid is composed partly of fibers continu- 

 ous with those of the ventral funiculus of the spinal cord (the direct pyra- 

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

 funiculus of the cord of the opposite side (the crossed pyramidal tract), 

 which decussate at the ventral portion of the medulla. The united 

 fibers can be traced upward to the pons, where they disappear from view. 



The lateral funiculus is composed of fibers continuous with those of the 



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