The ventral fasciculus ascends the cord and brain-stem lateral to the spino-thalamic tract. It forms the surface of the cord from the mid-lateral line nearly to the an terior roots of the spinal nerves. Running up the surface of the lateral column of the cord and medulla, it continues through the lateral part of the reticular formation of the pons, in the same lateral relation to the spino-thalamic tract, until it passes the root of the trigeminal nerve; there, it separates from the spino-thalamic tract, winds backward over the dorso-lateral surface of brachium conjunctivum, enters the superior medul lary velum of the cerebellum, decussates with its fellow of the opposite side and terminates in the cortex of the vermis superior cerebelli. The ventral spino-cerebellar fasciculus probably carries pain, temperature and localizing tactile impulses which, under normal conditions, are non-sensory; they excite coordi nating reflex impulses. But, if the impulses are very powerful, they excite the appropriate reflexes and then overcoming the synaptic resistance flow on to the conscious centers in the cere brum by way of the brachium conjunctivum and rubro-thalamic tract. The ventral spino-cerebellar fasciculus, alone, is called Gowers' tract by some anatomists.
The fasciculus is the superficial anterior and the deep lateral part of Gowers' tract. It forms a long, thin crescent in sections of the cord. By the anterior roots of the spinal nerves it is divided into an anterior and a lateral fascicu lus, like the vestibulo-spinal tract, which descends beneath it. The lateral spino-thalamic fasciculus is covered throughout the spinal cord by the ventral spino-cerebellar tract; but the anterior fasciculus follows the surface of the cord in front of the anterior roots, where it is intermingled with the anterior vestibulo-spinal tract. In the brain-stem, the spino-thalamic tract ascends medial to the ventral spino-cerebellar tract to the point of divergence in the pons at the level of the root of the trigeminal nerve; above the level of the trigeminal root the spino-thalamic tract continues into the tegmentum of the mid-brain, accompanied by the spino-tectal fasciculus; the latter ends in the quadrigeminal colliculi of the tectum, the former con tinues through the tegmentum and terminates in the ventral part of the lateral nucleus of the thalamus. The spino-thalamic tract carries impulses of pain, heat, cold, light touch, pressure touch, and tactile localization. Of these impulses the anterior spino-thalamic fasciculus carries only light touch and pressure touch, according to W. Page May; the lateral fasciculus con veys the remainder. Though all these impulses just enumer
ated traverse the spino-thalamic tract, each one travels through its own specific bundle and, as a result of localized lesion, may be lost without any of the others being affected, excepting only impulses of light touch and pressure. They are not lost be cause there is a second path for them, an uncrossed one, in the posterior column. This specificity of the lateral column is in accord with the law that second order afferent neurones form specific tracts.
Spino-olivary Fasciculus.—On the surface of the cord and just lateral to the anterior roots of the spinal nerves, there runs a small tract, triangular in section, commonly called the triangular-tract of Helwig. It is the custom at present to de scribe it as the spino-olivary fasciculus, because of the as sumption that it rises in the gray matter of the cord and terminates in the inferior olive of the medulla; but its origin, termination and function have not been positively determined.
Dorsal Spino-cerebellar Fasciculus (fasciculus spino-cere bellaris dorsalis, direct-cerebellar tract of Flechsig).—Both spino-cerebellar tracts are located in the lateral column of the spinal cord; hence, they are best distinguished by the adjectives ventral and dorsal. The dorsal spino-cerebellar tract forms the dorsal half of the lateral surface of the cord, as the ventral spino-cerebellar tract, assisted by the spino-olivary fasciculus, forms the ventral half of that surface. It covers the lateral pyramidal tract by its deep surface. Its dorsal border rests against the marginal tract of Lissauer, which separates it from the posterior roots of the spinal nerves. Below the second lumbar segment its absence allows the lateral pyramidal tract to come to the surface. The dorsal spino-cerebellar tract runs from the dorsal nucleus (Clarki) of the cord to the superior worm of the cerebellum. Its fibers are axones of vesicular cells in that nucleus. In the medulla, crossing over to the posterior column, it enters the restiform body and proceeds to the cortex of the superior vermis cerebelli on both sides. It conveys non sensory, sympathetic impulses, received, especially, from the viscera. In the dorso-lateral part of the spino-cerebellar tract is a small strand of fibers discovered by Horsley and Thiele in Igor, called the spino-vestibular tract. It rises in the lumbo-sacral region of the cord and, ascending along the surface of the dorsal spino-cerebellar tract to the medulla, winds inward dorsal to the restiform body and terminates in the nucleus of the vestibular nerve.