Matsyasana (Fish)

The thyroid and parathyroids in the throat area are massaged and toned.  Apex of the lungs get oxygen.  Pineal and pituitary glands are toned by the inversion of the upper body.  The chest is greatly expanded, aiding the correct posture.  Massages congested parts of the neck and shoulders.  The larynx and trachea are wide open.  The upper spinal nerves are nourished. Stretches the intercostal muscles of the ribs and builds the strength and elasticity of the lungs. Coming in and out of the pose strengthens the muscles of the neck.  Opens clear communication.

I sometimes tense my jaw in this pose, so I suggest to smile.  In fact, anytime you feel tense, just smile.  

Source: www.manasayoganyc.com