304 THE MEDULLA OBLONGATA. 



controlling, and in some cases arresting its action, and this is done through 

 influences propagated along the tubular structures of the posterior and an- 

 terior columns, which therefore are to be regarded, in this respect, as 

 commissures to the brain. 



OF THE MEDULLA OBLONGATA. 



The medulla oblongata is a conical body, lying between the spinal cord 

 Limits of the an< ^ ^ e brain. ^ * s generally understood to be bounded at 

 medulla obion- its upper portion by the pons varolii, but this is not a true 

 limit, since its structure extends through the pons varolii to 

 the crura of the brain. There is the same indefiniteness of limit as re- 

 spects its lower boundary, which is generally said to be marked by some 

 decussating fibres which appear on its front. Like the spinal cord, it 

 Its subdivis- has an anterior and posterior fissure, which divide it into two 

 ions. symmetrical lateral halves ; the former is a continuation of 



the anterior spinal fissure, the latter of the posterior, and ends in the ca- 

 lamus scriptorius above. The lateral halves thus produced are marked 

 by three grooves, producing four eminences, which pass under the follow- 

 ing names : 1st. The anterior pyramids ; 2d. The olivary bodies ; 3d. 

 The restiform bodies ; 4th. The posterior pyramids. The anterior fis- 

 sure is crossed about an inch below the pons varolii by decussating fibres, 

 and hence injuries on one side of the brain produce nervous effects on the 

 opposite side of the body. 



First. The anterior pyramids consist of white fibres originating near 

 The anterior tne decussating fasciculi. They have a compound structure, 

 pyramids. f or each contains fibres arising from the inner side of the op- 

 posite anterior column of the cord, and also fibres from its own side : 

 they pass through the pons varolii into the crus cerebri. From these 

 pyramids curved fibres pass round the olivary body, and are lost in the 

 restiform. They are called arciform fibres. 



Second. The corpora olivaria receive their name from their olive shape. 

 The olivary They are separated by a groove from the preceding in front, 

 bodies. an( j by another groove from the restiform bodies behind. Ex- 

 ternally, they are formed of white tubular tissue, which incloses a vesic- 

 ular mass, the olivary ganglion, which connects with the vesicular struc- 

 ture of the pons above, and that of the cord below. The fibres of these 

 ganglia are called the olivary tracts. They are continuous with the cen- 

 tral part of the medulla oblongata, passing behind the pyramids, extend- 

 ing upward along the posterior part of the crura cerebri to the optic thai- 

 ami and tubercula quadrigemina. The olivary bodies exist only in man 

 and the monkey tribe. 



Third. The restiform bodies are separated from the olivary by a 

 groove. They are continuous with the posterior and antero-lateral col- 



