Observations on the Autonomic Nervous System and on Vertebral and Neural Segmentation in Monkeys.

Abstract

SUMMARY. Rhesus monkeys normally have 29 precaudal vertebra, comprising 7 cervical, 12 thoracic, 7 lumbar, and 3 sacral segments. In 15 per cent. of 241 specimens a pair of small rib‐like processes articulated When the accessory thirteenth ribs are unequal with the first lumbar vertebra. in size, the right is usually larger than the left. Variations in the total number of thoraco‐lumbar vertebrae have been observed in only eight (3‐3 per cent.) of 241 specimens. In two the number was reduced to 18, and in six it was increased to 20, 13 rib‐bearing vertebrz being followed by seven non‐rib‐bearing lumbar vertebra The number of sacral vertebra was abnormal in only two of 168 specimens. In one there were four sacral vertebræ, in the other two. As a result of the attachment of the lumbo‐costal arches to the second lumbar vertebra, the first lumbar (thirteenth postcervical) vertebra is invariably intrathoracic in position. In the fully prefixed type of lumbo‐sacral plexus the femoral and obturator nerves are formed by branches of the third, fourth, and fifth lumbar roots, and the most cranial root of the sciatic is the fifth lumbar. In the fully postfixed type of lumbo‐sacral plexus the sixth lumbar root contributes to both the femoral and obturator nerves, and is also the most cranial root of origin of the sciatic. Intermediate arrangements occur in which the sixth lumbar root supplies the obturator and not the femoral nerve, and in which the uppermost root of origin of the sciatic is either the fifth or sixth lumbar. The prefixed type of plexus occurred in 59 (52.5 per cent.), the fully postfixed in 31 (28 per cent.), and intermediate arrangements in 22 (19.5 per cent.) of 112 dissections. Specimens with 13 pairs of ribs usually have a postfixed lumbo‐sacral plexus. Such a plexus also frequently occurs in animals with the more usual number of 12 pairs of ribs. The arrangement of the lumbo‐sacral plexus in anomalous specimens with an extra segment in the thoraco‐lumbar series suggests that the extra vertebra is formed by the duplication of the normal seventh lumbar (nineteenth postcervical) vertebra. A detailed description of the main sympathetic nerves in the rhesus monkey is provided. The caudal limit of emergence of white rami communicantes was the third lumbar root in 51, the fourth lumbar in 57, and the fifth lumbar in three of 111 specimens. There is a tendency for the caudal level to be the third lumbar root in a prefixed and to be the fourth lumbar in a postfixed plexus. The sacral autonomic nerves usually spring from the first and second sacral There is a possibility that some (twentieth and twenty‐first postcervical) roots. fibres of these nerves pass cranially in the hypogastric nerve‐trunks. The relations of vertebral levels to the levels of origin of the spinal nerves from the cord are described. The anatomy of the lumbo‐sacral plexus and of the sympathetic nerves in 12 other species of Primate is briefly stated.