Morphology of the Spinal Cord, Spinal Nerves, Caudal Plexus, Tail Segmentation, and Caudal Musculature of the Spider Monkey *

Spiml cord The spinal cord of the spider monkey has 37 segments consisting of eight cervical, thirteen thoracic, five lumbar, three sacral, and eight tcaudal segments. The number of oervical segments is always eight, but there is a considerable variation in the number of thoracic segments, depending upon the number of rib-bearing or thoracic vertebrae. Of ten specimens examined, six have been found with 13 pairs of ribs and four with 14 pairs. In the latter cases the lumbar segments were correspondingly reduced from five to four, the total ,number of the spinal segments remaining unchanged. Spider monkeys have a sacrum formed by the fusion of three sacral vertebrae, -and consequently have three sacrl segments of the spinal cord. The elongated caudal region of the spinal cord constitutes one of the ,distinctive features of the central nervous system of the spider -monkey. Some other mammals, e. g., the cat, may have a large -number of caudal nerves (7 or 8 pairs), but the dimension of the caudal cord is small by comparison with that of the spider monkey. In the spider monkey each segment in the caudal region is almost of