ON THE CIRCULATION’ THROUGH IT 179 
drawings are accurate to scale, though magnified for convenience of represen- 
tation ; and from these as well as from the calculated numbers, it will be seen 
that, when the limb was horizontal, the area of the internal calibre was more 
than three times as great as in the elevated position, and that, when it was depen- 
dent, the capacity of the tube was increased about sevenfold. 
The more we consider these facts, the more clear is it that they cannot be 
accounted for as merely mechanical results of diminution and increase of the 
pressure of the blood upon the arterial walls, in consequence of the different 
effects of gravity upon the fluid in the tubes in different positions. The arteries, 
METACARPAL ARTERY OF HORSE 
SECTION LIMB 
AFTER REMOVAL] ELEVATED 
AREA OF AREA OF AREA OF AREA OF 
CALIBRE CALIBRE CALIBRE CALIBRE 
12:5 40 132 276 
in any given state of contraction of their transversely arranged muscular fibres, 
are by no means disposed to yield readily in the lateral direction to increase, 
of pressure from within. This is evident from the fact that they are not in- 
creased in diameter by the successive strokes of the powerful muscular pump, 
the heart. The surgeon, when tying a large arterial trunk in its continuity, 
does not find, on clearing the vessel of its sheath with the point of his knife, 
that he is dealing with a body that swells at every pulse, but with one of unvarying 
dimensions. And, in the experiment on the metacarpal artery of the horse 
above referred to, no changes in the transverse measurements were noticed so 
long as the limb was maintained in any one position.! If any increase do occur 
length ; so that the material composing its wall was thinned out, not only in consequence of lateral 
expansion, but also to some slight degree through longitudinal stretching. 
’ Professor A. W. Volkmann, in his valuable work, Die Haemodynamik, relates experiments which 
he made by forcibly injecting water into portions of arteries removed from the body, proving that they 
yielded in the transverse direction even more than in the longitudinal to pressure from within. But 
Volkmann wrote before the discovery of the vaso-motor nervous system (he published in 1850), other- 
wise he would have been well aware that his experiments on dead animals left out of account altogether 
the most important element in this question, viz. the muscular contraction of the vessels as distinguished 
from mere elasticity. Volkmann admits that, when an artery is laid bare in the living animal, the only 
perceptible evidence of expansion of the vessel during pulsation is the tortuous form assumed by it 
at every pulse in consequence of longitudinal stretching, and that the transverse distension which he 
supposes to take place on theoretical grounds is inappreciable. Even in the dead vessel, he could 
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