Psychophysical experiments show that the perception of posture is to a
large degree affected by hitherto unknown graviceptors in the human t
runk. By remote control subjects move themselves radially along their
spinal axis over the horizontal platform of a rotating centrifuge unti
l they feel horizontal. Normal subjects then set the centrifuge axis o
n average at 22-28 cm caudal of the meatus, neuromectomized subjects a
t 45-55 cm. Hence the mass centroid of these receptors should be situa
ted near the last ribs. Evaluation of the residual faculties of parapl
egic patients lead to the conclusion that somatic graviception is medi
ated by two distinctly localized inputs, the first entering the spinal
cord at the 11th thoracic segment, and the second reaching the brain
cranial of the 6th cervical segment, presumably via the N. phrenicus o
r the N. vagus. The effect of the first named input is abolished after
bilateral nephrectomy. This proves that the kidneys affect gravity pe
rception. But whether they function like statoliths or in another way
cannot yet be decided. For the second input, however, the results show
unequivocally that it yields gravity information through the inertia
of a mass in the body. It is hypothesized that this mass may be that o
f the blood in the large vessels. This is corroborated by the effect o
f shifting blood craniad by means of positive pressure to the legs. It
is inferred that the inertial forces are measured by mechanoreceptors
in the structures that mechanically support the large vessels, rather
than by baroreceptors.