http://findarticles.com/p/articles/mi_m1134/is_6_109/ai_63290990 Is Rattlesnake Venom Evolving?
Natural History, July, 2000 by Steve Grenard
"Rattlesnake venom is not a simple poison. The snake's venom glands; located at the rear of the upper jaw and connected by ducts to its pair of hollow fangs, produce a complex brew of toxic peptides, polypeptides, and enzymes. In the venom, these toxins are combined in differing proportions that vary throughout a species' range and even during an individual snake's lifetime. Rattlesnakes harbor so many biochemical mixtures for venom that toxinologists who analyze the stuff confront a range of variations rather than a standard formula for each species.
Some of this variability seems to reflect recent changes in the venom of certain rattlesnakes, from the hemotoxic and proteolytic type (which affects blood and other tissues) to the neurotoxic type (which attacks the nervous system). The first type hasn't changed into the second; rather, the proportion of neurotoxins in the mix appears to have increased in some areas of the country. Consequently, victims may now receive a significant dose of both types of poison from a single bite."