Experimental demonstration of the entire course of four descending tracts 1 by a single alcoholic injection in the mid-brain of the cat

Abstract

By trephining a hole 2 cm. in diameter, 1 cm. anterior to the occipital crest and 2 cm. from the median line, and retracting the occipital pole of the cerebral hemisphere of the cat, the superior colliculus was exposed and about two drops of 95 per cent. alcohol injected into the nucleus ruber with a fine hypodermic syringe. The needle should enter the surface at about the middle of the colliculus and be directed slightly medially and at right angles to the long axis of the brain stem to a depth of 1 cm. After the usual two weeks, the animal was killed and the brain carried through the regular Marchi method.

As the accompanying illustrations show, it is possible in this manner to cause degeneration of the entire extent of four descending tracts, two of which are not easily recognizable on normal material and yet are functionally of great importance. These fiber bundles in the cat are situated in the same relative position as in man.

Although the exact course (especially peripherally) of the mesencephalic root of the trigeminal nerve has been much debated 2 , it is clearly evident that its exit through the pons is closely associated with the motor root of the trigeminal nerve.

The fibers frotin the nucleus of the medial longitudinal fasciculus (degenerated only on one side) and those from the tectum mesencephali (degenerated mastly on the contralateral side) occupy distinct regions throughout their entire course in the cat. Numerous other experiments in which we have limited the lesion to the tectum of the mid-brain have failed to show that any fibers from this region enter what is strictly the medial longitudinal fasciculus. The lower termination of the tectospinal fasciculus has been variously given 3 .