Liver is the unyielding viscus

The liver thrives by orderly reaching and is averse to being repressed or constrained and to hyperactivity [of its yáng aspect]. The unyielding nature of the liver is mainly seen in liver qì: mental stimulus gives rise to impatience, agitation, and irascibility with flights of anger, which is called excess of liver qì. Conversely, insufficiency of liver qì gives rise to susceptibility to fright and fear. The liver and gallbladder stand in exterior-interior relationship, and the unyielding nature is a combined manifestation of both organs.

See also sinew.