Chapter 13 Lecture Notes (Free, Downloadable pdf)
Dr. Zoval’s General, Organic, and Biochemistry Textbook
(Free, Downloadable pdf)
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Horns, such as those of the impala in this image, are made up of a protein (keratin) that covers live bone.
Source: Wikimedia Commons, Author: Muhammad Mahdi Karim, GNU Free Documentation License http://www.gnu.org/licenses/old-licenses/fdl-1.2.html