The impulses traversing these five paths must evoke their appropriate sensations in the cortex. Painful and pleasurable sensations arise in the thalamus. Temperature impulses excite sensations in the thalamus, also; but they are often painful or pleasant rather than thermal (Brain, Vol. 34).
Destructive lesions of the lateral nucleus, according to size and location, cause exaggerated response to painful and pleasur able stimuli, or a degree of anesthesia and ataxia on the opposite side of the body.
7. The nucleus of the pulvinar (Fig. 56) is an important one. It receives about 20 per cent. of the optic fibers and gives rise to a corresponding number of the corticipetal fibers in the optic, or thalamo-occipital radiation; hence, a lesion of the pulvinar impairs vision. It is continuous with the lateral nucleus.
The white matter of the thalamus includes, first, the stratum zonale of the superior and medial surface, which is derived from the occipito-thalamic radiation and the lateral root of the optic tract, and the zona lateralis of the lateral surface; and, second, the interior fibers, a part of which form the internal medullary lamina. Into the thalamus enter the medial fillet, the spino thalamic tract, a small part of the medial longitudinal bundle, the brachium conjunctivum cerebelli and perhaps some other tegmental fibers, all carrying common sensory impulses; they end chiefly in the lateral nucleus, whence the cortical fillet proceeds to the cerebral cortex. The thalamus also receives fibers from the special sense paths, from the optic, auditory, olfactory, and the gustatory, and gives rise to fibers that con tinue in those paths to the special sense areas of the cortex. It is also known that the thalamus is entered by a considerable number of corticifugal fibers, especially through the occipito thalamic and temporo-thalamic radiations. Besides the inter nuncial fibers that associate. the different thalamic nuclei together, the thalamus has either a genetic or terminal relation to the following fasciculi: 1. The columna of the fornix, having pierced the thalamus, descends to the corpus mammillare and terminates in its medial nucleus, whence the bundle of Vicq d'Azyr, the mammillo thalamic bundle, rises and ascends to the thalamus. It ends in the anterior nucleus.
2. The stria medullaris thalami (Fig. 87) from the hippocam
pus and from the region of the olfactory triangle, terminates partly in the tectum, but chiefly in the nucleus habenulm and from this nucleus the fasciculus retroflexus or habenulo-peduncu laris originates and descends to the interpeduncular nucleus. Both "one" and "two" belong to the olfactory paths.
3. The ventral stalk of the thalamus, connected with its ven tro lateral part, is a compound funiculus situated below the lentiform nucleus; it contains afferent and efferent fibers and is often called ansa peduncularis. It is divided by the nucleus interansalis into (a) an upper stratum and (b) a lower stratum. (a) The upper stratum, the ansa lenticularis, is made up of thalamo-striate and of hypothalamo-striate fibers, joining lateral nucleus and hypothalamic nucleus with globus pallidus; and these fasciculi are intermingled with strio-fugal fibers. The strio-fugal fibers connect the globus pallidus with the thalamus, red nucleus, hypothalamic nucleus and substantia nigra. (b) The lower stratum, the inferior peduncle of the thalamus, contains cortico-thalamic fibers which connect the temporal and insular cortex with the lateral nucleus of the thalamus (Villiger).
The common afferent thalamo-cortical fibers already de scribed on pp. 102-103 form the parietal and frontal stalks of the thalamus. They are axones of the lateral nucleus which terminate in the central, the posterior part of the frontal and the middle of the cingulate gyri; they are intermingled with cor tico-thalamic fibers. They constitute the cortical fillet.
4. The parietal stalk, comprising five specific bundles, ascends through the occipital part of the internal capsule; it contains the common sensory fibers going to the posterior central gyrus and the adjacent part of the paracentral gyrus (the true somzesthetic area) as well as many fibers to the motor and psychic motor area.
5. The frontal stalk of .the thalamus rises in the anterior part of the lateral nucleus; it traverses the frontal part of the capsule and terminates in frontal cortex. Like that part of the parietal stalk which ends anterior to the central sulcus, the frontal stalk is probably concerned with the automatic control of the dis charge of motor impulses from the emissive motor center.