Experimental light and electron microscopic studies were carried out to determine the length of parallel fibers in the cat cerebellar cortex. Using a fine surgical knife, vermal and hemispheral folia were cut perpendicular to their long axis. The animals were sacrificed 1-10 days after the operation. Sections of the transected folia were then stained with a Fink-Heimer procedure. The resulting degeneration appeared as fine dots that extended lateral to the lesion, as predictable from the course of the parallel fibers. Densitometer readings indicate that the density of degeneration declines gradually lateral to the lesion. The specificity of the silver impregnation was checked by processing silver stained sections for electron microscopy. This confirmed the location of the silver precipitate on degenerating parallel fibers. The pattern of parallel fiber degeneration in the molecular layer has a trapezoidal configuration centered on the lesion. The shorter parallel fibers are located at the base of the molecular layer and extend for 5 mm. The parallel fibers become progressively longer as they approach the pial surface where they attain a maximum length of 7 mm. Our studies suggest that in folia longer than 7 mm parallel fibers are 6 mm long on the average. In addition, it was determined on Golgi sections that the average center-to-center distance between en passant boutons of individual parallel fibers is 5.2 mum. The data indicate that an average parallel fiber, 6 mm long, forming approximately 1100 boutons, may synapse with each Purkinje dendritic tree it traverses.

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The cellular composition of the primate claustrum was analyzed using serially sectioned Golgi impregnated neurons. The tissue used in this study was embedded in a soft resin mixture and cut with 25 mm long glass knives. The resin embedding allowed the sections to be cut serially at a thickness of only 3 micrometers. A camera lucida was employed for drawing the cellular processes from selected impregnated neurons; these drawings were later incorporated into a single composite picture of the neuron. Three types of neurons were observed in the primate claustrum. The largest of these neurons (Type 1) had a cell body and spine-laden dendritic arborization that varied in size and shape according to the neuron's position in the claustrum. The axons of Type I neurons were successfully impregnated in 25-day-old animals and were found to form collaterals within the claustrum. The collaterals from the axons of these cells appeared to leave the claustrum through both the external and extreme capsules. A second neuron found in the claustrum (Type II) had a round cell body with smooth beaded dendrites which radiated in all directions. The axon of the Type II neuron appeared to give off numerous collaterals that were not observed to leave the claustrum. A third type of neuron (Type III) had a small pear shaped cell body and a sparse dendritic tree. The axon and its collaterals appeared to remain within the dendritic circumference of the Type III neuron.

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