In a monograph published in 1783, Monro made the following points:
• The
brain was enclosed in a nonexpandable case of bone
• The substance of the
brain was nearly incompressible
• The volume of the blood in the cranial
cavity was therefore constant or nearly constant
• A continuous outflow of
venous blood from the cranial cavity was required to make room for the
continuous incoming arterial blood
Thus, appeared in the literature for the
first time the hypothesis that the blood circulating in the cranium was of
constant volume at all times.
George Kellie of Leith, another Scot and himself a former pupil of Monro,
studied the amount of venous blood in the brain of humans and animals that had
died of various causes, including drowning, hanging, and exsanguination. He
noted that there was no significant difference in the amount of the brain venous
blood in these various circumstances, a finding that supported Monro's
observations. In his experiments on animals that had been bled to death, Kellie
noted that the brain had maintained its usual appearance and was not pale or
drained of blood. He also noted that the amount of blood in the cerebral vessels
of dogs given lethal doses of prussic acid was not affected by gravitation. Kellie
was fascinated that congestion of cerebral vessels was not noted in some
instances in which it might be most suspected. He remarked on postmortem
observations on the bodies of two pirates who were hanged at Leith and were
dissected by Monro and himself. Although extracranial tissues were congested and
engorged, the intracranial vessels were not. Kellie similarly noted that:
• The
brain was contained in, and exactly filled, an unyielding case of bone
• The
brain itself was only minutely compressible
• It was unlikely that any
circulating fluid could be withdrawn from or any overflow could be introduced
within the cranial cavity without simultaneous equivalent replacement or
displacement. |