492 



PHYSIOLOGY 



CHAP. 



from its different portions, and spread like a fan into the centrum 

 ovale. They are grouped into three principal bundles. The anterior 

 bundle emerges at the frontal end of the thalamus, rniis through 

 the anterior segment of the internal capsule, and spreads to the 

 cortex of the frontal lobe. The posterior bundle arises in the 

 pulvinar and corpus geniculatum externum, follows an antero- 



posterior course, crosses the 

 hindmost segment of the 

 external capsule, and spreads 

 out to the cortex of the 

 occipital lobe (optic radia- 

 tion of Gratiolet). The in- 

 ferior bundle starts from the 

 mesial and ventral parts of 

 the thalamus, runs obliquely 

 lateralwards, passes along 

 the ventral surface of the 

 lenticular nucleus, and 

 finally ends in the cortex 

 of the temporal lobe and 

 iusula (ansa peduncularis of 

 Gratiolet). 



It is important to deter- 

 mine the origin, course, and 

 termination of the optic 

 nerves, and their connection 

 with the mid -brain, thala- 

 mus, and cerebrum. The 

 fibres that make up each 

 optic nerve arise from the 

 ganglion cells of the retina. 

 They emerge from the eye- 

 ball a little mesial to the 

 posterior pole, and enter the 

 cranium through the optic 

 foramen. Directly after 

 entering the skull both optic 

 nerves unite in the chiasma, in which more than half the fibres 

 decussate, and thence pass to the posterior part of the optic 

 thalamus or pulvinar under the name of the optic tract. 



Experimental physiology and pathological anatomy show 

 clearly that each optic nerve contains two distinct bundles of 

 fibres : one direct, which remains on the side in which it takes 

 origin, and a crossed bundle, which decussates in the chiasma to 

 pass to the other side. The fibres of the direct bundle come from 

 the external or temporal third of the retina ; those of the crossed 

 bundle from the two inner or nasal thirds. The dividing line 



Fio. 247. Thalamencephalon and its relations. Frontal 

 section through grey commissure. 'XTestnt.) a, 

 frontal portion of lateral ventricle ; b, its inferior 

 horn ; c, third ventricle ; d, Sylvian fissure ; e, 

 optic tract ; /, gyms hippocampi. 1, lamina 

 medullaris interns of optic thalamus ; 2, lamina 

 medullaris externa ; 3, internal ; 4, external ; 5, 

 superior nucleus of optic thalamus ; 6, caudate 

 nucleus ; 6', its lower end ; 7, 7', lenticular nucleus ; 

 8, claustrum ; 9, external capsule ; 10, internal 

 capsule; 11, pes pedunculi ; 12, substantia nigra ; 

 13, stratum dorsale of regio subthalamica ; 14, zona 

 incerta ; 15, nucleus' of Luys ; 16, anterior end of 

 red nucleus ; 17, great commissure. 



