708 THE XER VOL'S SYSTEM 



The optic commissure is formed by the union and partial decussation of the 

 optic tracts of opposite sides. Each optic tract may be traced backwards from the 

 connnissure, around the crus cerebri, to the region of the corpora geniculata 

 (page 710). 



The lamina cinerea is a thin plate of gre}' matter which may be seen from 

 the base of the brain by pulling the optic conmiissure slightly l)ackwards. It is 

 continuous in front with the grey matter of the olfactory convolutions of the 

 frontal lobes, and between the latter it is connected to the rostrum of the corpus 

 callosum by the basal white commissure. Laterally, it is continuous with the 

 anterior perforated space. It forms the anterior sloping part of the floor of the 

 third ventricle. 



Nerve-tracts in the substance of the thalamencephalon and prosen- 

 cephalon. — The systems of fibres which are connected with the optic thalanms 

 may be tirst considered; the principal of these are: (a) The posterior commis- 

 sure; (b) the corona radiata of the ojitic thalamus; and (c) the bundle of Vicq 

 d'Azyr. 



(a) The posterior commissure contains a tract of fibres which passes from the 

 optic thalamus to the tegmental region of the mesencephalon on the opposite side; 

 but beyond this, nothing is known with any degree of certainty. It is said to 

 transmit fibres from the fillet and from the posterior longitudinal bundle to the oppo- 

 site side. Fibres are also said to proceed via this commissure from the ganglion 

 habenulae and the pineal body to the oculo-motor nucleus of the opposite side. It 

 is therefore a bundle of decussating fibres derived from various sources, and prob- 

 ably does not contain any true commissural fil)res. 



(6) The corona radiata of the optic thalamus is formed of a system of fibres 

 which converge to the thalamus from tlie cortex of the frontal, parietal, occipital, 

 and temporal lobes, and it also receives a small Ijundle from the cortical part of the 

 olfactory tract. Portions of this system, namely, the fibres from the occipital lobe 

 (posterior peduncle of the thalamus), and from the tem])oral lobe (inferior peduncle), 

 require special notice. The posterior peduncle of the thalamus (optic radiation) 

 passes backwards in the white matter of the hemisphere in company with a great 

 bundle of sensory fibres to the occipital lol le. It is further described in connection 

 Avith the optic nerve (page 746). The inferior peduncle may, for the sake of 

 simplicity, be described with the ansa peduncularis, as it forms an important 

 constituent of that body. 



The ansa peduncularis may be exposed from the base of the brain by dissecting 

 the optic tract from the surface of the crus cerebri. It is a band of fibres which 

 winds round the ventral surface of the crusta just at the transition of the latter 

 into the inner capsule. Its principal constituents are the ansa lenticularis and 

 the inferior peduncle of the thalamus. The ansa lenticularis proceeds from the 

 medullary laminae of the lenticular nucleus and passes transversely inwards 

 immediately below the globus pallidus. It then enters the subthalamic region, and 

 without doubt passes into the mesencephalon, but its exact course cannot be followed 

 witli certainty. The inferior peduncle of the thalamus is })artly formed by a 

 sagittally-directed bundle which passes l)ackwards through the sul)stance of the 

 thalamus l)etween the anterior pillar of the fornix and the bundle of Vicq d'Azyr. 

 This bundle is joined by fibres from the superior surface of the thalamus (stratum 

 zonale). The fibres from the stratum zonale pass from the superior to the internal 

 surface of the thalanms, and finally, reaching the inferior surface of that body, 

 join the sagittal l)undle first mentioned to form the inferior peduncle; the latter 

 now passes outwards below the ansa lenticularis and ends in the cortex of the tem- 

 poral lobe. 



(c) The bundle of Vicq d'Azyr passes from the anterior tubercle of the thala- 

 mus to the nucleus of the corpus all)icans. It might be regarded as a constituent 

 of the corona radiata — the nucleus of the corpus allncans being considered as 

 modified cortex. 



The white matter of the cerebral hemispheres contains the following systems of 

 fibres: (a) The corona radiata; (b) commissural or interhemispheric fibres; (c) 

 association or intrahemispheric fibres. These three systems of fibres are disposed 

 in more or less defined tracts which can be unravelled by dissection, but in certain 



