354 ANATOMY OF THE RABBIT 



ending there and part continuing to the superior colHculi 

 of the midbrain. The lateral geniculate body is a relay 

 station for visual impulses to the cerebral cortex, the fibres 

 which convey from the eye impulses producing conscious 

 visual experiences being among those which terminate here. 

 The fibres to the superior colliculus of the mammal are con- 

 cerned entirely with the production of reflex adjustments. 

 (m) On the ventral surface, the optic chiasma (chiasma opti- 

 cum), forms a conspicuous median cross-like elevation, the 

 posterior portion of which is traceable into the optic tracts, 

 the anterior portion into the bases of the second cranial, 

 or optic nerves. Thus the optic tracts are simply the direct 

 continuation of the optic nerves after they have crossed 

 in the chiasma. 



While all mammals (and only mammals) have a certain proportion 

 of optic nerve fibres which do not cross in the chiasma, the number of 

 these is at a minimum in the rabbit, in which the eyes have an extreme 

 lateral direction. In mammals generally, the number of uncrossed 

 fibres is roughly proportional to the amount of overlapping of the 

 fields of vision of the two eyes. 



(n) The hypophysis, or pituitary body, lies immediately behind 

 the optic chiasma (cf. p. 134). It is a somewhat elongate, 

 rounded, glandular organ attached to the base of the 

 brain by a slender stalk. 



On account of its enclosure by the walls of the hypo- 

 physeal fossa, and of the relative weakness of the stalk 

 connecting it to the brain, the hypophysis is commonly 

 detached in removal of the brain from the skull, in which 

 case a slit-like aperture representing the ventral portion of 

 the third ventricle is exposed. 



The meninges fuse in a collar-like ring round the transition between 

 the hypophyseal body and the infundibular stalk. Thence the dura 

 continues so as to form a complete lining for the bony hypophyseal 

 fossa, fused externally with the periosteum and over its whole inner 

 surface with the fibrous capsule of the gland. Thus the subdural and 

 subarachnoid spaces do not extend round the gland. 



(o) The tuber cinereum is a small elevation of grey matter 

 appearing on the ventral surface after the removal of the 

 hypophysis. It is the base of attachment of the infundi- 



