THE BRAIN 



333 



orsal funiculi of the spinal cord. The axons of the neuroblasts forming 

 these receptive nuclei decussate through the reticular formation chiefly as 

 internal arcuate fibers and ascend to the thalamus as the median lemniscus. 



There are developed from neuroblasts of the alar plate other nuclei the 

 axons of which connect the brain stem, cerebellum and fore-brain. Of these the 

 most conspicuous is the inferior olivary nucleus. 



The characteristic form of the adult myelencephalon is determined by the 

 further growth of the above-mentioned structures. The nuclei of origin of the 



Mid-bn 



Lobuln of verm is 



dulla 

 ob/onycitti 



Lateral lobe of 

 Cereb ellum 



Flocculus 



FIG. 319. Dorsal views of four stages in the development of the cerebellum. A, of a 13.6 mm. em- 

 bryo (His); B, of a 24 mm. embryo; C, of a no mm. embryo; D, of a 150 mm. embryo. 



cerebral nerves, derived from the basal plate, produce swellings in the floor of the 

 fourth ventricle which are bounded laterally by the sulcus limitans. The terminal 

 nuclei of the mixed and sensory cerebral nerves lie lateral to this sulcus. The 

 enlarged cuneate and gracile nuclei bound the ventricle caudally and laterally 

 as the cuneus and clava. The inferior olivary nuclei produce lateral rounded 

 prominences and ventral to these are the large cerebrospinal tracts or pyramids. 

 The Metencephalon. Cranial to the lateral recesses of the fourth ventricle 

 the cells of the alar plate proliferate ventrally and form the numerous and rela- 

 tively large nuclei of the pons. The axons from the cells of these nuclei mostly 



