THE NERVOUS SYSTEM. 309 



opticus, with two tubercles on its posterior border, called external 

 and internal corpora geniculata ; a yellowish mass with white striae, 

 called corpus striatum ; a pale semicircular band, called tcenia 

 semicircularis, between the two ; and a plexus of vessels, called 

 plexus choroides. The floor of the cavity has various promi- 

 nences : one called hippocampus major, or cornu ammonis, which 

 is a prolongation of the posterior extremity of the mesolobe in 

 the inferior cornu; and a small one of the same kind in the 

 posterior cornu, called hippocampus minor, or ergot; another 

 called corpus Jlmbriatum. Under the septum is another long 

 white body called thejbrnix, with a few transverse lines called 

 lyra at its lower surface, extended over a third ventricle, which 

 is placed exactly in the centre, and to which an opening leads 

 at each side of the fornix from the corresponding lateral ven- 

 tricle. The anterior extremity of the fornix divides into two 

 pillars, which diverge and run down to two projections at the 

 base of the brain, called corpora mammillaria, pisiformia, or 

 albicantia, between which is a grey triangular plate, called pons 

 Tarini: its posterior extremity does the same, and each pos- 

 terior division itself divides into two, one of which is the corpus 

 fimbriatum ; and between this and the thalamus opticus exists a 

 chink through which the pia mater, or innermost covering of the 

 brain, enters into the third ventricle and unites with the plexus 

 choroides, which is, in fact, a plexus of vessels, connected by 

 cellular membrane, called, in this part of the body, pia mater. At 

 the posterior extremity of the fornix are seen four eminences, 

 called corpora quadrigemina ; the two higher and larger called 

 nates, or c. q. anteriora ; the two smaller and lower called testes, 

 or c. q. posteriora ; and, before them all, is a grey body, called 

 pineal gland, generally containing grit, and attached to the brain 

 by two medullary prolongations only, which run to the thalami 

 optici. Behind and below the corpora quadrigemina, is a fine 

 layer of transverse greyish fibres, called valve of Vieussens, which 

 is formed by three converging bands, named processus a cerebello ; 

 ad testes. Three bands of white matter, called commissures, run 

 transversely over the third ventricle, establishing more commu- 

 nication between the two halves of the cerebrum. The anterior 

 part of the floor of the third ventricle is formed by the upper 

 surface of a small grey body, called tuber cinereum, which runs 

 downwards in a conical form under the name of infundibulum, and 

 ends in a little mass called pituitary gland, and lodged in the fossa 



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