OF ANIMAL BODIES. 
101 
ment afterwards in the small arteries, and thereby to ob¬ 
struct the circulation: and it was also necessary that this 
extreme tenuity should be compensated by multitude; for, 
a large quantity of chyle (in ordinary constitutions, not 
less, it has been computed, than two or three quarts in a 
day) is, by some means or other, to be passed through 
them. Accordingly, we find the number of the lacteals 
exceeding all powers of computation; and their pipes so 
fine and slender, as not to be visible, unless filled, to the nak¬ 
ed eye; and their orifices, which open into the intestines, 
so small, as not to be discernible even by the best micro¬ 
scope. Fourthly, the main pipe, which carries the chyle 
from the reservoir to the blood, viz. the thoracic duct, be¬ 
ing fixed in an almost upright position, and wanting that 
advantage of propulsion which the arteries possess, is fur¬ 
nished with a succession of valves to check the ascending 
fluid, when once it has passed them, from falling back. 
These valves look upward, so as to leave the ascent free, 
but to prevent the return of the chyle, if, for want of suffi¬ 
cient force to push it on, its weight should at any time 
cause it to descend. Fifthly, the chyle enters the blood 
in an odd place, but perhaps the most commodious place 
possible, viz. at a large vein near the neck, so situated with 
respect to the circulation, as speedily to bring the mixture 
to the heart. And this seems to be a circumstance of 
great moment; for had the chyle entered the blood at an 
artery, or at a distant vein, the fluid, composed of the old 
and new materials, must have performed a considerable 
part of the circulation, before it received that churning in 
the lungs, which is probably, necessary for the intimate 
and perfect union of the old blood with the recent chyle. 
Who could have dreamed of a communication between the 
cavity of the intestines and the left great vein near the 
neck ? Who could have suspected that this communication 
should be the medium through which all nourishment is 
derived to the body? or this the place, where, by a side 
inlet, the important junction is formed between the blood 
and the material which feeds it? 
II. We postponed the consideration of digestion, lest it 
should interrupt us in tracing the course of the food to the 
blood; but, in treating of the alimentary system, so prin¬ 
cipal a part of the process cannot be omitted. 
Of the gastric juice, the immediate agent by which that 
change which food undergoes in our stomachs is effected, 
we shall take our account, from the numerous, careful, and 
varied experiments of the Abbe Spallanzani. 
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