MICROSCOPIC DRAWING AND ENGRAVING, 
convenient to view it, first, with a simple magnifying-glass, having 
a power of 15 to 20, so as to obtain a general view of the vessels 
and of the circulation; even with this small power the observer will 
be filled with astonishment at the magnificence of the spectacle, 
especially if a strong light is thrown upon the lower side of the 
tongue. To imagine a geographical map to become suddenly 
animated, by their proper motions being imparted to all the 
rivers delineated upon it, with their tributaries and affluents, 
from their fountains to their embouchures, would afford a most 
imperfect idea of this object, in which is rendered plainly visible, 
not only the motion of the blood through the great arterial trunks, 
and thence through all their branches and ramifications to the 
capillaries, but also its complicated vorticular motions in the 
glands, its return through the smaller ramifications of the veins to 
the larger trunk veins, and its departure thence en route for the 
heart. Such is the astonishing spectacle, circumscribed within a 
circle having the diameter of the 120th of an inch, magnified, 
however, 400 times in its linear, and therefore 160000 times in its 
superficial dimensions, which has been daguerreotyped by Messrs. 
Donne and Foucault, and which is reproduced on the same scale 
in fig. 39, p. 65. 
83. The arteries are distinguishable from the veins very readily, 
by observing the direction in which the blood flows, its velocity, 
and their comparative calibre. In the arteries the blood flows 
from the trunk to the branches, its course is marked by the 
arrows in fig. 39, where t is a trunk-artery entering near the 
lowest point of the field of view; the arrows show the course of 
the blood passing into the principal branches, 1, 2, and 3, from 
which it flows into all the smaller arterial ramifications. The 
course of the blood in the veins, on the contrary, is from the 
branches to the trunk, from whence it finds its way back to the 
heart. The arteries, moreover, are of less calibre than the veins, 
and consequently the blood flows in them with greater velocity. 
The greater arteries are accompanied by a greyish flexible cord, 
which can be perceived, but not without some attention; it passes 
along the sides of the artery : this cord is only a nerve. 
As the ramifications of the arteries are multiplied they are 
diminished in calibre, and merge at length in the capillaries, from 
which they are scarcely distinguishable, the latter being equally 
indistinguishable from the smaller veins. As these conduits of 
the blood diminish in diameter, the red corpuscles at length so 
completely fill them, that they can only move in them one by one, 
and they can be thus seen following one another at perceptible 
intervals. If the microscope be directed to that part of the edge 
of the tongue, which is within the limits of the hole made in the 
104 
