THE BRAIN. 129 



and thrown into numerous transverse folds, CP, which hang 

 down into the ventricle, and between the layers of which lie the 

 blood vessels of the choroid plexus of the ventricle. 



The mid-brain, BM, thickens on its floor to form the crura 

 cerebri. Its roof grows out laterally into a pair of hollow ovoid 

 processes, the optic lobes: and its cavity persists as the 

 aqueductus Sylvii. 



The fore-brain, BF, becomes the thalamencephalon of the 

 adult : its cavity becomes the third ventricle, which by thicken- 

 ing of its walls to form the optic thalami is reduced to a vertical 

 cleft, very narrow from side to side. Its floor is produced down- 

 wards and backwards into a hollow sac-like diverticulum, the 

 infundibulum, I, in connection with which is the pituitary body. 

 In front of the infundibulum is a transverse ridge projecting 

 into the ventricle, and formed by the roots of the optic nerves. 



The roof of the fore-brain remains thin : a little behind the 

 middle of its length the pineal body, PN, arises as a median 

 hollow diverticulum. Fig. 29 ; this is formed at the spot where 

 the final closure of the neural tube took place, and is at first 

 directed backwards : in the later stages it grows forwards and 

 forms a rounded vesicle connected with the brain by a long 

 pigmented stalk : when the skull develops it cuts off the vesicle 

 from the stalk, the former remaining as a small rounded body 

 outside the skull, while the stalk persists as a slender pigmented 

 tract within the cranial cavity. 



In front of the pineal body, and at the anterior end of the 

 fore-brain, the roof is thrown into folds which hang down into 

 the ventricle forming a choroid plexus, CP', similar to that in 

 the medulla. 



The anterior end of the fore-brain grows forwards as a median 

 thin-walled cerebral vesicle, from which at a slightly later stage 

 the cerebral hemispheres, CH, arise as a pair of hollow out- 

 growths ; the foramina of Monro being the apertures of com- 

 munication being the lateral ventricles or cavities of the 

 hemispheres, and the third ventricle. The anterior ends of the 

 hemispheres grow forwards as the olfactory lobes, which become 

 fused together in the median plane. 



The peripheral nervous system. The cranial nerves and the 

 dorsal roots of the spinal nerves are formed from the deeper or 

 nervous layer of the epidermis. They arise as lateral outgrowths 

 from the edges of the neural plate, and may be recognised at a 



