Gross spinal anatomy and limb use in living and fossil reptiles

The spinal quotient (S.Q.) is an osteologically defined estimate of the enlargement of the spinal cord at limb levels over that at interlimb levels. It is an efficient predictor of limb use in living reptiles and birds and may be used to predict limb function in fossil vertebrates. Among living reptiles, this ratio of limb to interlimb innervation is greatest in arboreal genera, followed by terrestrial sprawlers, aquatic forms, and undulatory forms. Birds show a wide range of brachial S.Q. values that are roughly commensurate with flight ability. S.Q. values for the manipulative forelimbs of some dinosaurs fall well above those of locomotory limbs. Dinosaur hind-limb values are either well within ranges predicted by living reptiles and birds (most taxa), or highly inflated (stegosaurs, sauropods). This inflation may be the result of presence of a glycogen body similar to that of birds. In no case does the lumbosacral S.Q. support the presence of a “sacral brain.”

The meningeal arrangement of the avian lumbosacral spinal cord

The meningeal arrangement of the lumbosacral cord of Gallus is comparable to that of mammals. The meninges are formed by three lamellae: inner, intermediate, and outer. The arachnoid space, between the outer and intermediate lamellae, is wide and filled with a trabecular network, except in the region of the glycogen body and lobes of Lachi. This region is invaded by the glycogen body cells which also disrupt the inner and intermediate lamellae. A similarity between this region and the cerebellomedullary cistern of mammals is postulated, and the meaning of the presence of glycogen in the outer cerebrospinal fluid compartment is also discussed.