400 MEDULLA OBLONGATA. 



Its internal surface, as stated, is in contact with the brain_, 

 und united to it by the immense number of vessels passing 

 from it into its substance. 



MEDULLA OBLONGATA. (Fig. 16.) 



The medulla oblongata, or racliidian bulb, is situated upon 

 the cuneiform process of the occipital bone, occupying its 

 concave surface, and forming the upper portion of the spi- 

 nal marrow. It extends from the atlas, or first vertebra, 

 to the pons Varolii, being represented as from " fourteen to 

 fifteen lines in length, nine lines in breadth, 3-nd six in 

 thickness." Its shape is conical, the base being above, the 

 apex below. Like the spinal marrow, it has a median fis- 

 sure in front and behind, dividing it into two lateral and 

 symmetrical parts, each of which is again divided by three 

 grooves, presenting upon its surface four eminences: the 

 corpora pyramidalia, corpora olivaria, corpora restiformia, 

 and posterior pyramids. 



According to Mr. Solly, each lateral half of the spinal 

 marrow consists of antero-lateral and posterior columns; 

 the antero-lateral being again divided into two portions, 

 not by any anatomical separation, but, he says, "physio- 

 logically;" the half or two-thirds of the anterior part 

 being the motor tract, and corresponding to the corpora 

 pyramidalia and olivaria of the medulla oblongata, each of 

 which sends fibres to the cerebrum and cerebellum, while 

 the balance of the antero-lateral column, with the poste- 

 rior, is for sensation. 



The anterior median fissure of the medulla oblongata is 

 broader but not so deep as the spinal, of which it is a con- 

 tinuation. It is lined by the pia mater, and is connected 

 by transverse commissural fibres. About an inch below 

 the pons, this fissure is obstructed by the fasciculi of the 

 anterior columns of the spinal marrow, crossing or decus- 

 sating each other at the lower end of each pyramid. This 

 decussatlon explains why injuries upon one side of the 

 brain affect the opposite side of the body; but as all the 

 fibres do not cross over, this further explains the occa- 



