THE MEDULLA OBLONGATA. 285 



cord; and the flattened base joins the pons. On the anterior 

 surface, a transverse groove marks the boundary between the 

 medulla and pons. The medulla is a bilateral organ composed 

 of symmetrical halves (Figs. 85 and 86). In the interior, the two 

 halves are united by both gray and white matter in the raphe, but 

 on the surface they are partially separated by the anterior and 

 the posterior median fissures (fissura mediana anterior and f. 

 m. posterior). These fissures are continued through the spinal 

 cord, but neither extends the whole length of the medulla. The 

 anterior median fissure is interrupted in the lower part of the 

 medulla by the crossing of two large tracts of fibers, forming the 



Fig. 90. Section of embryonic medulla. Length of back, 9.1 mm. 

 (Gordinier and Minot after His.) 



RL. Rhomboid lip. Ts. Tractus solitarius. X. Vagus nerve. XII. Hypoglossal nerve. 



decussation of the pyramids; while only through the lower half 

 of the medulla does the posterior median fissure extend. 



Origin. The medulla oblongata is developed from the myel- 

 encephalon of the embryo (Figs. 90 and 118). The myelen- 

 cephalic floor and walls thicken and form the greater part of the 

 medulla. Inferiorly, the roof undergoes some thickening; but 

 it stretches out into a single layer of epithelium, superiorly, which 

 is continuous at its upper end with the inferior medullary velum 

 of the cerebellum. 



Ventricle. The common cavity of the posterior brain vesicle 

 persists in the mature brain as the fourth ventricle (Figs. 118, 

 90 and 86). The fourth is, therefore, the ventricle of the adult 

 rhombencephalon (see page 313). 



