474 



THE NEBVOtTS SYSTEM 



the central canal of the spinal cord, its cephalic portion forms the 

 cerebral vesicles. The former portion retains an approximately 

 equal caliber throughout ; the latter soon presents three character- 

 istic dilatations, the three primary cerebral vesicles, whose walls 

 respectively develop the prosencephalon, mesencephalon, and rJiom- 

 bencephalon. 



The first and third of these primary cerebral vesicles soon 

 subdivide into two each, five secondary vesicles being thus formed. 

 These five vesicles are the telencephalon, diencephalon, mesencepha- 

 lon, met encephalon } and myelencephalon. 



The following table exhibits the relation of the several primi- 

 tive vesicles to each other as well as to the gross divisions of the 

 central nervous system : 



(I. TELENCEPHALON 

 (endbrain) 

 II. DIENCEPHALON 

 (interbrain or 



' ENCEPHALON 

 (brain) 



NEURAL CANAL 



II. MESEN- 



CEPHALON 



MYELON 

 (spinal cord) 



III. MESENCEPHALON 

 % 



(midbrain) ( <^) 



III. RHOMBEN- f IV. METENCEPHALON 



CEPHALON I (hindbrain) 



(rhomboid \ 

 brain) ( 



V. MYELENCEPHALON 

 (afterbrain) 



The encephalon develops the various portions of the brain, the 

 cranial nerves and their ganglia. The myelon forms the spinal 

 cord, the spinal nerves and their ganglia. The sympathetic 

 nerves and ganglia are outgrowths from the cerebro-spinal nerve 

 roots. 



The following lists will show more definitely the destination of 

 the several portions of the encephalon and the parts of the brain 

 formed from each of the cerebral vesicles. 



1. From the Telencephalon, The anterior end of the third ven- 

 tricle, the lateral ventricles, the foramen of Monroe, the cerebral 

 hemispheres with their cortex or pallium, the olfactory bulb and 

 its tracts (sometimes called the rhinencephalon), the corpora stri- 

 ata, the corpus callosum, and the fornix. 



2. From the Diencephalon. The third ventricle, the optic nerve 

 and retina, the optic tracts (pars optica hypothalmi), the optic 



