Destruction of any of the above sensory paths causes diminu tion or loss of the especial variety of impulse which travels that path. Destruction of the posterior white columns produces loss of muscular sensations and gives rise to ataxia. Interrup tion of Gowers's tract (spino-thalamic and ventral spino-cere bellar tracts) abolishes pain and temperature sensations while touch is not much affected.
2. Special Sensations Impulses producing the sensations of smell, sight, hearing and taste are carried from the respective organs of sense to the brain by the following nerves: The olfactory, the optic, the auditory, and the glossopharyngeal and intermediate nerves.
Olfactory Path (Figs. 151 and 2t).—Impulses of smell origi nate in the upper third of the nasal mucous membrane. They run through the olfactory nerves to the second layer in the bulb, where they are transferred to the dendrites of the mitral and brush cells. By the axones of these cells they are carried back ward through the olfactory tract to the third order neurones whose cell-bodies are located in the cortex of the tract, the olfac tory triangle, the anterior perforated substance and the septum pellucidum. The third order neurones form the olfactory stria'; they conduct the impulses to the cortical center, the hippocampal formation, and to certain reflex centers, the thalamus, the amyg dala, the nucleus habenulx, etc. The lateral stria is most direct; it bears the olfactory impulses around the anterior perforated substance to the uncus hippocampi. The medial stria (the stria Lancisii) reaches the cortical center by encircling the corpus callosum; it conveys the impulses by way of the gyrus subcallosus, gyrus supracallosus, gyrus subsplenialis and f as ciola cinerea, and fascia dentata to the hippocampus. The intermediate stria comprises four bundles of fibers, only one of which terminates in the cortical center of smell; the other three end in subcortical centers concerned with the reflex functions of smell. (1) The olfacto-hippocampal bundle carries impulses from the olfactory triangle, perforated substance and septum pellucidum to the cortical center through the body and crus of the fornix. (2) The olfacto-amygdalate bundle bears impulses from similar third order stations up to the thalamus and, then, forming the stria terminalis, takes them on to the nucleus amyg dalm; some of these cross through the anterior commissure.
(3) The olfacto-habenular bundle conducts olfactory impulses from perforated substance and septum pellucidum to both nuclei habenulac through the stria medullaris thalami and the corn missura habenularum; also to quadrigeminal colliculi. (4) The olfacto-mesencephalic bundle carries impulses from the cortex of the olfactory tract to tuber cinereum, mammillary body, tegmentum of mid-brain, pons and medulla and through fibers that join the medial longitudinal bundle, even into the spinal cord. In the brain-stem and spinal cord, these olfactory im pulses probably enter motor nuclei and excite reflexes.
Optic Path (Figs. 152 and 153).—Impulses of sight originate in the rods and cones of the retinae and traverse three or more series of neurones to the terminal nuclei of the optic tracts; namely, the rod and cone, the bipolar, and the ganglionar neu rones. The axones of the last form the optic nerves and the visual part of the optic tracts. From the right halves of both retinae and from the left halves of both, impulses run through the corresponding tract to the lateral geniculate body and the ,pulvinar of the thalamus; also to the superior quadrigeminal colliculus. The latter produces ocular and pupillary reflexes. From the lateral geniculate body and pulvinar the thalamo occipital radiation carries the impulses through the pars occipi talis of the internal capsule to the half-visual center in the cu neus, gyrus lingualis and the pole of the occipital lobe. Impulses from the nasal halves of the retina decussate in the optic chi asma; those from the temporal halves, for the most part at least remain on the same side, but a few may cross through the quad rigeminal colliculi and brachia superiora. Impulses from the nasal half and from the temporal half of the macula lutea are conducted equally by both optic tracts. Hence destruction of one tract causes hemianopsia, preserving the vision in the corre sponding half of each visual field, and also diminishes the acute ness of macular vision in both eyes.