376 ZOOTOMY. 



optic thalamus is a thin white band, the peduncle of the 

 pineal body (p.pri) : the two peduncles jgin with one 

 another at the posterior boundary of the thalami, and are 

 continued into a flat band about a quarter of an inch long, 

 which ends at the posterior boundary of the cerebral hemis- 

 pheres in a rounded vascular mass : this is the pineal 

 body : it rests on the dorsal surface of the optic lobes. 



518. The posterior commissure (p.co), seen by 

 turning aside the stalk of the pineal gland : it is a delicate 

 transverse band of white fibres, somewhat smaller than the 

 anterior commissure, connecting the thalami with one 

 another just below the junction of the peduncles of the 

 pineal body. 



519. The optic lobes, or corpora quadrigemina 

 (<?./), two pairs of rounded elevations, composed externally 

 of grey matter, and lying immediately above the crura 

 cerebri, and bounded by the optic thalami in front and 

 by the cerebellum behind. The anterior larger pair are 

 regularly ovoidal in shape, and are called the nates (o.l 1 } : 

 the posterior pair, or testes (<?./ 2 ), are transversely elon- 

 gated, and lie at a lower level than the nates. 



520. By removing the remainder of the temporal lobe 

 of one side, the optic tracts (p.tr, 486) are seen to curve 

 round the antero-external corner of each optic thalamus, 

 from the chiasma, and then to pass backwards and inwards 

 to the nates. 



521. The corpus geniculatum (c.gii), a small rounded 

 elevation on the external surface of the optic thalamus, 

 immediately posterior to the optic tract. 



LXIL Cut away the whole of the central part of the 

 cerebellum,, leaving only its points of attachment, 

 at the sides, to the medulla oblongata. Note : 

 22. The fourth ventricle (z'-4), a flat, lozenge-shaped 



