The structure and development of avian lumbosacral specializations of the vertebral canal and the spinal cord with special reference to a possible function as a sense organ of equilibrium.

The avian lumbosacral vertebral column and spinal cord show a number of specializations which have recently been interpreted as a sense organ of equilibrium. This sense organ is thought to support balanced walking on the ground. Although most of the peculiar structures have been described previously, there was a need to reevaluate the specializations with regard to the possible function as a sense organ. Specializations were studied in detail in the adult pigeon. The development of the system was studied both in the pigeon (semiprecocial at hatching) and in the chicken (precocial). Specializations in the vertebral canal consist of a considerable enlargement, which is not due to an increase in the size of the spinal nervous tissue, but to a large glycogen body embedded in a dorsal rhomboid sinus. The dorsal wall of the vertebral canal shows segmented bilateral dorsal grooves, which are covered by the meninges towards the lumen of the vertebral canal leaving openings in the midline and laterally. This results in a system of lumbosacral canals which look and may function similar to the semicircular canals in the inner ear. Laterally these canals open above ventrolateral protrusions or accessory lobes of the spinal cord which contain neurons. There are large subarachnoidal cerebrospinal fluid spaces, lateral and ventral to the accessory lobes. Movement of this fluid is thought to stimulate the lobes mechanically. As to the development of avian lumbosacral specializations, main attention was given to the organization of the lobes and the adjacent fluid spaces including the dorsal canals. In the pigeon the system is far from being adult-like at hatching but maturates rapidly after hatching. In the chicken the system looks already adult-like at hatching. The implications derived from the structural findings are discussed with regard to a possible function of the lumbosacral specializations as a sense organ of equilibrium. The adult-like organization in the newly hatched chickens, which walk around immediately after hatching, supports the assumed function as a sense organ involved in the control of locomotion on the ground.