626 THE NERVOUS SYSTEM 



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 con- 

 nected with the optic tract, see figure 406. The posterior nucleus, lying just 

 below the pulvinar, is a small mass and is connected with the cortex of the 

 interior parietal convolution. The cells of the 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 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 thalami, the majority of which form synapses about 

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

 tinct ending of the nerve fiber, collaterals are given off which establish physio- 

 logical connection with the nuclei. 



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

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

 involve the conscious perception of sensory stimuli wherever they may 

 arise. Flechsig even claims that in the thalamus there are definite points 

 of sensory localization corresponding to every sensory point in the periphery 

 of the body (including the special senses). The 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 

 its nuclei. It is probable, however, that extensive co-ordinations of afferent 

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

 walking, riding, writing, speaking, etc., are possibly reflexes co-ordinated 

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

 of walking. 



Corpora Geniculata. The corpora geniculata form prominences on 

 each side of the peduncles of the cerebrum just ventral to the anterior corpora 

 quadrigemina. There are two on each side, the external or outer, and median 

 or inner. The external corpus geniculatum is at the side of the crus and 

 appears to be a swelling on the lateral division of the optic tract and actually 

 receives terminations of the optic fibers, thus constituting a way-station in 

 the optic conduction paths. Similarly the median appears to be the ter- 

 mination of the median division of the optic tract, from which it receives 

 some fibers, figure 406, but it is more intimately connected with the auditory 

 tracts, forming a way-station between the lateral fillet and the auditory cor- 

 tical center, figure 414. 



