MICROSCOPIC DKAWING AND ENGRAVING. 



convenient to view it, first, with a simple magnifying-glass, having 

 a power of 1 5 to 20, so as to obtain a general view of the vessels 

 and of the circulation ; e^en 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 aflluents, 

 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 



