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these arise the dorsal roots of the spinal nerves, and the 

 5th, 7th, 8th, 9th and loth cranial nerves (which are peri- 

 pheral nerves, having been formed from the neural crest). 

 During later development the nerves become attached 

 to spinal cord and brain. 



Describe (1) the Brain and Spinal Cord, and (2) the Sym- 

 pathetic Nervous System. 



1. By constriction of the primary vesicle, the primitive 

 fore-brain or cerebrum (prosencephalon), the mid-brain or 

 mesencephalon, and the hind-brain (rhombencephalon or 

 metencephalon) are established. Bulgings of the side walls 

 of the fore-brain project forwards and form the two cerebral 

 hemispheres, from which anteriorly the olfactory lobes 

 arise. The cavities of the cerebral hemispheres are the 

 lateral ventricles. The fore-brain becomes the thalamence- 

 phalon ; its cavity is the third ventricle (communicating 

 with the lateral ventricles by the foramen of Monro), from its 

 thin roof arises the small pineal body or epiphysis (a primitive 

 sense organ), its thickened side walls form the optic thalami, 

 and an outgrowth from its floor is the infundibulum (without 

 or with lobi inferiores and a glandular saccus vasculosus). 



The infundibulum is associated with the hypophysis 

 (derived from the stomodaeum), the two forming the 

 glandular pituitary body, which influences or regulates 

 the growth of bone ; its superfunction causes that 

 hypertrophy of the bones (skull and limbs) known as 

 acromegaly. 



On the floor of the thalamencephalon transverse ridges 

 arise ; one of these becomes the optic chiasma, the others 

 form the anterior and posterior commissures connecting 

 the optic thalami. 



The thick roof of the mid-brain is bulged out (in most 

 Craniata) to form the two optic lobes ; and the side walls 

 and floor constitute the thick crura cerebri. The contracted 

 cavity of the mid-brain is the iter or aqueduct of Sylvius, 

 which is continuous with the cavities of the optic lobes. 

 The thickened front portion of the roof of the hind- brain 

 becomes the cerebellum, which is a special centre for that 

 co-ordination of muscular movement known as equilibration. 

 The hind- brain, beneath and behind the cerebellum, forms 

 the medulla oblongata or bulb ; it projects forwards (resti- 

 form bodies) along each side of the mid-brain, it is con- 

 tinuous behind with the spinal cord, and its cavity is the 

 fourth ventricle ; across its ventral surface, and connected 



