DEVELOPMENT OF THE NERVOUS SYSTEM 



741 



floccular fissure; when the two lateral walls fuse, the right and left floccular fissures 

 join in the middle line and their central part becomes the post-nodular fissure. 



On the ventricular surface of the cerebellar lamina a transverse furrow, the 

 incisura fastigii, appears, and deepens to form the tent-like recess of the roof of the 

 fourth ventricle. The rudiment of the cerebellum at first projects in a dorsal 

 direction ; but, by the backward growth of the cerebrum, it is folded downward and 

 somewhat flattened, and the thin roof-plate of the fourth ventricle, originally 

 continuous with the posterior border of the cerebellum, is projected inward toward 

 the cavity of the ventricle. 



The Mid-brain or Mesencephalon. The mid-brain (Figs. 650 to 654) exists for a 

 time as a thin-walled cavity of some size, and is separated from the isthmus rhomb- 

 encephali behind, and from the fore-brain in front, by slight constrictions. Its 

 cavity becomes relatively reduced in diameter, and forms the cerebral aqueduct 

 of the adult brain. Its basal laminse increase in thickness to form the cerebral 

 peduncles, which are at first of small size, but rapidly enlarge after the fourth month. 



Ganglion hdbemUce 



Optic s'alk 

 Hypophysis cerebri 



FIG. 652. Exterior of brain of human embryo of five weeks. (From model by His.) 



The neuroblasts of these laminse are grouped in relation to the sides and floor 

 of the cerebral aqueduct, and constitute the nuclei of the oculomotor and trochlear 

 nerves, and of the mesencephalic root of the trigeminal nerve. By a similar 

 thickening process its alar laminse are developed into the quadrigeminal lamina. 

 The dorsal part of the wall for a time undergoes expansion, and presents an internal 

 median furrow and a corresponding external ridge; these, however, disappear, 

 and the latter is replaced by a groove. Subsequently two oblique furrows extend 

 medialward and backward, and the thickened lamina is thus subdivided into the 

 superior and inferior colliculi. 



The Fore-brain or Prosencephalon. A transverse section of the early fore-brain 

 shows the same parts as are displayed in similar sections of the medulla spinalis 

 and medulla oblongata, viz., a pair of thick lateral walls connected by thin floor- 

 and roof-plates. Moreover, each lateral wall exhibits a division into a dorsal or 

 alar and a ventral or basal lamina separated internally by a furrow termed the sulcus 



