MYELENCEPHALON. 



423 



forward almost to the plane of the floor, and a transverse section 

 of the vesicle in that region presents the form of a very broad 

 capital V with the roof epithelium stretching between the two 

 arms and converting the letter into a triangle. Transverse sec- 

 tion through the lower half of the myelencephalon at the third 

 or fourth week shows an elongated ellipse with a dorso-ventral 

 major axis. 



Internal Surface (Fig. 125). The lateral wall, in both upper 

 and lower regions, presents the longitudinal groove which sepa- 

 rates the ventral and dorsal zones. The median ventral groove 

 persists throughout and the lateral grooves are represented by the 



jar 



Fig. 126. Transverse section of the medulla from an embryo of eight weeks. 

 (McMurrich after His.) 



av. Spinal tract of the trigeminal nerve, jr. Substantia reticularis. 01. Olivary nucleus 

 of medulla, sf. Tractus solitarius. tr. Restiform body. XII. Hypoglossal nerve. 



sulcus limitans and inferior fovea of the fourth ventricle (Fig. 96). 

 The low eminence situated between the median and each lateral 

 groove becomes the eminentia medialis. 



External Surface (Figs. 125 and 126). On the exterior sur- 

 face of the myelencephalon, along the ventral border of the 

 dorsal zone, there appears very early an oval bundle of de- 

 scending fibers, called the solitary tract. It is composed of 

 axones from the geniculate, glossopharyngeal and vagus gan- 

 glia, and constitutes the root-fibers of the intermediate, the ninth 

 and tenth pairs of nerves. At about the same time axones 

 from the semilunar ganglion form a bundle of descending 



