HEAD AND NECK OF THE HORSE 



195 



It is the custom to group together, under the common term 

 hypo-thalamus, a number of structures that lie ventral to the thalamus. 

 These are — the optic tracts and chiasma, the hypo-thalamic tegmental 

 region, the mammillary body, the tuber cinereum, the hypophysis and 

 the lamina terminalis. 



Each 02)tic tract (tractus opticus), in the form of a conspicuous 

 white band, curves round the lateral part of the cerebral peduncle to 

 meet its fellow at the optic chiasma on the ventral aspect of the brain. 

 To the naked eye it appears to commence in the lateral and medial 

 geniculate bodies ; but microscopically its fibres are connected with the 

 anterior colliculus and the thalamus, as well as with the geniculate 



White matter 

 Lateral ventricle. 



Caudate nucleus. ^ 



Corpus callosum. 



Septum pellucidum. 



Choroid plexus. 

 Fornix. 



Claustrum. " 

 External capsule. ' 

 Lentiform nucleus. 



ird ventricle. 



Internal capsule.' 



^. Anterior commissure. 



Columns of fornix. Optic recess. Optic chiasma. 

 Fig. 93.— Transverse section of the brain at the level indicated by D in Fig. 89 



(looking forward). 



bodies. It is, indeed, the custom to regard the tract as having two 

 roots— a medial root connected with the medial geniculate body, and a 

 lateral root ending in the thalamus, lateral geniculate body and the 

 anterior colliculus. The fibres of the medial root are commissural, inas- 

 much as they cross in the optic chiasma and become continuous with 

 the fibres of the medial root of the opposite side of the brain. The 

 fibres of the lateral root, on the other hand, are partly continued into 

 the optic nerve of the same side, and partly cross at the chiasma into 

 the nerve of the opposite side. 



The term hypo-thalamic tegmental region is applied to that part of 

 the tegmentum of the cerebral peduncle upon which the thalamus 

 rests. 



