An albuminous body, insoluble in water, but soluble in
   dilute solutions of salt. It is present in the red blood corpuscles
   united with haematin to form haemoglobin. It is also found in the
   crystalline lens of the eye, and in blood serum, and is sometimes
   called crystallin. In the plural the word is applied to a group of
   proteid substances such as vitellin, myosin, fibrinogen, etc., all
   insoluble in water, but soluble in dilute salt solutions.