The spinal roots in the sloth (Choloepus hoffmanni peters)

In the lower half of the thoracic region in Choloepus hoffmanni , a didactyl sloth, which possesses 23 dorsal vertebrae, the spinal roots assume a rostralward direction. This situation raises difficult problems in early embryological development as it already prevails — although slightly altered — in the 100 gm foetus. The myelinated nerve fiber diameters in the ventral roots are smaller than in the cat. This conforms to the general rule that “red” and slow muscle fibers are supplied by thinner motor fibers than the “white” fast muscles, because in the sloth the whole of the musculature is of the former type. The γ efferent system is well developed. The sympathetic preganglionic fibers are thought to be mostly unmyelinated. From the fiber spectrum in the dorsal roots it appears that the muscle spindle afferents are also of smaller diameter than in the cat. The cuscle spindles, the sympathetic chain and the aortic origin of the intercostal arteries are described.