This study is designed to better understand the mode of lymph transport, particularly through the extrinsic pumping by external compression of the lymph vessel. The pressure‐diameter relationship of lymphatic segments isolated from the canine thoracic duct was examined using a laser optical micrometer measurement system. Results revealed that the thoracic duct displayed a high extensibility or compliance in the physiological pressure range, yet became progressively less so with increasing internal pressure. The calculated incremental circumferential modulus of the thoracic duct under physiological pressure (range of 2 to 6 cm H
$_{2}$
O) showed values ranging from 1.2 × 10
$^{4 }$
to 3.61 × 10
$^{5}$
dyn/cm
$^{2}$
. At a pressure of 35 cm H
$_{2}$
O, the modulus reached a limiting value of approximately 6.0 ×10
$^{6}$
dyn/cm
$^{2}$
. In the physiological pressure range, the relative wall thickness (
$h/R_\mathrm{o}$
) of the canine thoracic duct was approximately 3.5%, which was much lower than that reported for canine arterial segments and similar in value to that of the canine jugular vein. In conclusion, the pressure‐diameter curve of the canine thoracic duct was shown to resemble that of venous vessels. However, the circumferential elastic modulus of the thoracic duct wall was lower than the moduli of veins, proving that lymphatics are more compliant than veins. This suggests lymph flow in the thoracic duct may be better promoted by external compression of the lymphatic vessel.
