ON ANIMAL HEAT. 
565 
the elastic property of the bronchial passages is called into action. 
Indeed, wherever elasticity is found, when exercised, the same 
effects invariably result. Muscular contraction, by which the 
fibres composing the muscles are, by their own peculiar nature, 
thrown into a state physically analogous to the coats of the 
arteries during their dilatation, is likewise a fertile source of caloric. 
33. But we have convincing proof that the elastic tissues of 
the body are capable of producing warmth. As the arteries 
maintain their elasticity after death, if we take, for convenience 
sake, a portion of the aorta of any large animal, the horse, for 
example, and close one end by tying it round a cork, and fix it 
to some weighty article, attach strings to the open end, in order 
that the warmth of the operator’s hands may not interfere with the 
experiment, fill the artery with water at the temperature of' the 
atmosphere to within an inch or two of the mouth, let fall into the 
water a delicate thermometer, note the temperature, and then by 
pulling the strings so as to call into play the elastic properties of 
the artery, we raise the mercury of the thermometer in a few 
seconds. But the small arteries piercing the coats of the aorta, 
by allowing the transudation of the fluid employed, produce a 
considerable degree of evaporation, which interferes with the cor- 
rectness of the illustration ; for, on performing the experiment, it 
will be noticed that soon after the operator ceases to exert the 
elasticity of the artery, the fluid acquires a lower temperature than 
it before possessed. To obviate this source of error we may 
employ a tube of Indian-rubber, fixed and treated in the same 
way, and the increase of temperature will become more marked 
and lasting. In less than half a minute, by moderately calling 
into play the elastic nature of the gum, we can raise the mercury 
of the thermometer 4° or 5° Fahr. 
34. Now, when it is taken into consideration that the elastici- 
ty of the arteries is incessantly at work throughout the whole 
bulk of the frame, it will cease to be a matter of surprise that 
warmth is generated. The extension of the arterial coats takes 
place both in length and in the direction of their diameter, but 
their elongation is much more considerable. Poiseuille invented 
an instrument for measuring arterial dilatation, which he found was 
about 2 V of the arterial capacity. Hunter let a horse bleed to 
death, and found that the aorta had contracted to the extent of 
more than -io of its diameter, the iliac artery £th, the crural ar- 
tery £d, and that arteries of the thickness of the radial in man 
were completely closed. 
35. It cannot, then, be denied that the arterial tissue plays an 
important part in the generation of vital heat. But, at the 
same time, it is also probable that the property of elasticity found 
vol. xvi. 4 G 
