108 NERVOUS SYSTEM 



alar and basal laminae. When the tube is opened up these become laid out hori 

 zontally, and form the mesial and lateral portions of the floor of the fourth ventricle. 

 The lateral grooves bounding the alar and basal laminae are represented by the 

 superior and inferior fovea of the adult brain ; the basal lamina becomes the 

 trigonum hypoglossi and eminentia teres, and the alar lamina the ala cinerea, the 

 tuberculum acusticum, and locus coeruleus. 1 The apex of the fourth ventricle 

 reaches at first to the cervical bend i.e. the beginning of the spinal cord. The 

 closed part of the medulla oblongata is a later formation produced by the neural 

 laminae closing in again (His), presumably owing to the great development of 

 the ganglionic masses in the alar laminae which form the clavate and cuneate nuclei. 

 The obex is the remains of the ependymal roof of this portion of the hind-brain. 



At a stage when the alar laminae still stand vertically their dorsal borders 

 become recurved and form what are known as the rhombic lips (fig. 144B). The 

 two folds of the lips fuse with one another, and form marginal swellings which 

 run in front into the cerebellar lamina. They become ultimately the restiform 

 bodies (third month), and share in the formation of the cerebellum (fig. 145). At 

 an early stage the afferent fibres of the vagus and glossopharyngeal nerves 

 which have grown in from the corresponding ganglia form a conspicuous bundle 



(funiculus solitarius) on the 

 ^-,^ . .__ ^ .. ^ _,...-'-h"misph e re gurface j ugt ven tral to the 



rhombic lip. It represents the 

 primary dorsal column of 

 the cord in this region. In 



~,emin. teres each rhombic lip a mass of 



.,, neuroblasts develops which 



tub. Rolando- - - ...- 



corresponds to the dorsal horn 

 in the cord, and from this 

 -fun.gradiis mass the cuneate nucleus and 



.. I 



substance of Kolando are de- 



I IF" 



veloped. In the mantle zone 

 of the alar and basal laminae, 



FIG. 147.-CEBEBELLUM, MEDULLA OBLONGATA, AND FOUBTH cloge to fa Q j^^ agpe ct, definite 

 VENTBICLE IN A FCETUS OF THE FIFTH MONTH. (From 



Kollmann's Entwickelungsgeschichte.) groups ol neuroblasts constitute 



the nuclei of the cerebral 



nerves ; but more superficially there are large numbers of scattered neuroblasts, 

 which persist as the nerve-cells of the formatio reticularis. Again, close to the 

 middle line on each side, but separated from it by a lamella of the reticular 

 zone, a large group of neuroblasts forms the rudiment of the corpus dentatum of 

 the olive. 



The reticular formation of the medulla oblongata is produced by a great 

 expansion of the reticular zone associated with the development of the nerve- 

 paths connected with the neuroblasts of the rhombic lips and olivary nuclei ; 

 the mesial raphe, like the ventral commissure of the cord, is partly formed by 

 these fibres crossing the middle line. As the reticular formation is gradually 

 added to, the funiculus solitarius becomes deeply buried. Its position in the 

 adult indicates the original surface of the embryonic medulla oblongata. The 

 last formations to appear are the pyramids as expansions of the reticular structure 

 mesial to the olivary nuclei. As they develop, they displace the olives laterally, 

 while at the same time the ventral median fissure is formed as a recess between 

 them. 



1 According to Wilson (Jour, of Anat. and Phys. voL.xl.), the alar lamina is represented by the area 

 postrema of Eetzius only, and the subdivision of the floor of the fourth ventricle described in the text 

 is not the primary one, which is triple, as in some lower forms. 



