The circulation in the three orders of vessels in
the mesentery, is exceedingly uncertain, from the
difficulty of keeping this membrane in precisely the
same situation. The least variation in the degree
of tension, produces changes in the velocity and
direction of the motion of the blood. And it is
impossible to avoid this : pins tear the membrane ;
the peristaltic movements of the intestine disturb
it. By the mere act of placing or replacing the
mesentery under the microscope, a circulation, or a
modification of the circulation, is produced, which
did not exist before.
At this moment I have the mesentery of a dead
frog in the field of my microscope. By waiting a
little time, all motion of the blood ceases. By
altering the situation of the membrane, a various
kind and degree of motion of the blood, which
might be called a circulation, is produced.
In fact I have long ceased to place the slightest
reliance upon the circulation as seen in the mesentery,
in physiological experiments of any delicacy.
The circulation of the web, on the other hand,
is unequivocal under judicious management, the
arrangement of the toes o and of the membrane
remaining accurately the same.
CHAPTER III.
ON THE INFLUENCE OF THE BRAIN AND SPINAL
MARROW UPON THE CIRCULATION.
This question has long engaged the attention
of physiologists. It particularly occupied the
attention of Haller, his followers, and his opponents.
Spallanzani, Fontana, and Whytt, were amongst
the first to submit the question to the test of experiment,
by removing or irritating the brain or
spinal marrow, and observing the effect upon the
actions of the heart.
More recently, the question has been investigated
experimentally by Legallois, Dr. Philip,
Mr. Clift, M. Flourens, M. Brachet, &c.
In the subsequent account of the experiments
of these various authors, and of my own, I shall