x. Ascending sensory tracts from the spinal cord.
a. The path for the conduction of impulses of touch, temperature and pain from the trunk and the extremities.
Neurone I: The impulse is conveyed from the periphery to the ganglion-cells within the spinal ganglion and thence to the spinal cord by the posterior roots. The latter enter the spinal cord and end within the gray substance.
Neurone II: Origin within the gray substance of the spinal cord. The fibres pass, as the axones of commissure-cells, by way of the anterior gray commissure to the opposite lateral column and form the tractus spino-thalamicus, which higher up joins the medial fillet and with it ends within the thalamus.
Neurone III: Origin within the thalamus. Course to the cerebral cortex, in part direct by way of the internal capsule, and in part after traversing the lenticular nucleus. Cortical ending within the area of somatic sensibility.
The conduction• of impulses of contact or tactile sensibility is not limited to the spino-thalamic tract, but takes place also through the long tracts of the posterior columns.
b. The path for the conduction of the from the trunk and the extremities.
Neurone I: The impulse is conveyed, as in the case of those of touch, temperature and pain, first to the spinal cord. The fibres enter as posterior roots, do not, however, end within the gray substance of the spinal cord, but ascend within the posterior column to the medulla oblongata, where they first find their ending within the nucleus gracilis and cuneatus.
Neurone II: Origin within the posterior column nuclei; course, after decussation, as medial fillet to thalamus and there end.
Neurone III: Origin within the thalamus. Course to cortical somatic sensory area. The conduction of muscle-sense is not only by way of the posterior column nuclei and the medial fillet, but also shares the tracts passing to the cortex by way of the cerebellum, that is by the tractus spino-cerebellaris ventralis et dorsalis. From the cerebellum, the conduction passes through the superior cerebellar peduncle to the thalamus and thence to the cerebral cortex.
It is to be noted, that tracts lead to the cerebellum also from the posterior column nuclei.
The partly uncrossed (muscle-sense or deep sensibility) and partly crossed (pain and temperature) conduction of sensibility within the spinal cord, explains the peculiar disturbances of sensibility in hemilesions of the cord, as manifested in the Brown-Sequard symptom-complex. In hemilateral lesions of the spinal cord, we find, on the same side as the lesion, paralysis in consequence of interruption of the descending motor paths, and disturbances of the deep sensibility or muscle-sense in consequence of the involvement of the ascending paths of the posterior column and of the spino-cerebellar tracts ; while, on the side opposite the lesion, are found disturbances of superficial sensibility, pain and temperature impulses in consequence of the crossed path of the tractus spino•thalamicus.
Further, the fact that the conduction of sensibility, especially of muscle-sense, also takes place by way of the cerebellum, supplies the explanation of those pathological dis turbances, which we designate as ataxia or errors of coordination, since the impulses proceeding from the muscles and articulations are no longer transmitted to the cerebellum in consequence of the lesion of the posterior column tracts.
2. Sensory Tracts of the Cerebral Nerves.
a. The path for the impulses of touch, temperature and pain from the integument of the head (with the exception of the occipital region and certain areas of the external ear—supplied respectively by the occipital and great auricular nerves), further, from the conjunctiva, the mucous membranes of the nasal fosse, of the mouth and tongue, of the palate, of the pharynx, etc., lies in the trigeminus, the glossopharyngeus, or the vagus.
b. The path for the impulses of orientation and the face lies probably in the trigeminus; that from the larynx probably in the vagus.