THE OPTIC THALAMI 547 



matter are known as the nuclei of the thalamus, and are six in number. They 

 are called the anterior nucleus, the median nucleus, the lateral nucleus, the 

 ventral nucleus, the pulvinar, and the posterior nucleus. The corpora geni- 

 culata are also closely associated with the optic thalamus. The anterior 

 nucleus is composed of large nerve cells which receive the terminations of 

 axones of cells of the corpora mammillaria at the base of the brain (bundle of 

 Vicq d'Azyr). There they meet the fibers of the fornix, which establish 

 a relation between this tubercle of the thalamus and the hippocampal con- 

 volutions. The median nucleus is connected by its axones with the cortex 

 of the island of Reil and the second and third frontal convolutions. The 

 lateral nucleus is quite large and lies against the internal capsule, into which it 

 sends fibers. It is connected with the central convolutions. The ventral 

 nucleus lies beneath the preceding; it is relatively small. It is connected with 

 the cortex of the frontal lobe and with the operculum, the central convolutions, 

 and the supramarginal gyrus. The fifth nucleus, known as the pulvinar, forms 

 the posterior tip of the thalamus, and is connected with the optic tract. The 

 posterior nucleus, lying just below the pulvinar, is a small mass and is con- 

 nected with the cortex of the interior parietal convolution. The cells of the 

 optic thalamus are thus seen to be connected with a large area of the cerebral 

 cortex. The axones spread out in a great fan in the corona radiata, the 

 thalamus sending more fibers to the cortex than are received from it. 



The collections of nerve cells in the optic thalamus are shown by anatomical 

 investigations and by methods of physiological degeneration to be on the 

 pathway of ascending or afferent nerve tracts. Large masses of sensory fibers 

 pass through the optic thalami, the majority of which form synapses about the 

 nerve cells in the thalamus. Even in those cases where there is no distinct 

 ending of the nerve fiber, collaterals are given off which establish physiological 

 connection with the nuclei. 



The optic thalamus is thus closely connected with large areas of the cortex. 

 It must at least form an important relay in all those activities which involve 

 the conscious perception of sensory stimuli wherever they may arise. Flechsig 

 even claims that in the optic thalamus there are definite points of sensory 

 localization corresponding to every sensory point in the periphery of the body 

 (including the special senses). The optic thalamus also receives fibers from 

 various parts of the cerebral cortex, thus establishing a double relation with 

 this region. 



Owing to the difficulty of those operations which establish isolation of the 

 thalamus, it is not clear to what extent reflex actions may take place through 

 these nuclei. It is probable, however, that extensive coordinations of afferent 

 impulses may be mediated by the nuclei of the thalami. Such activities as 

 walking, riding, writing, speaking, etc., are possibly coordinated reflexes 

 through the optic thalami, perhaps with the assistance of the medulla in the 

 case of walking. 



