Spinal Roots in the Sloth

Abstract

IN the course of an investigation intended to relate the size of the myelinated motor nerve fibres to the speed of the muscles in Choloepus hoffmanni Pet.¹, in which the whole musculature is ‘red’ in character, we were struck by the unusual arrangement of the spinal roots. It is generally assumed that the backward obliquity of the spinal roots in adult mammals is due to a differential rate of growth of the spinal cord and of the vertebral column. In man, this obliquity increases from the second cervical root to the filum terminale. In cat, dog and sheep, the obliquity of the D₁₃–L₂ roots is reduced, but more caudally the roots resume their strong backward direction. In the ox, a similar alteration in the course of the spinal roots leads to a situation where the tenth and eleventh dorsal roots are at right angles to the spinal cord. In the horse, which has 18 dorsal vertebrae, the oblique run of the spinal roots starts from D₂ decreases from D₁₁ to D₁₇ and then increases again₂.