42 
which most of us have observed flicking across the eye of domestic fowls. The 
eyes of snakes are now covered by a transparent scale, which is shed each time 
the creature casts its skin. The ear-drum and middle ear cavity have also gone, 
and snakes appear to have no hearing for sound vibrations in the air. These are 
all mutations which would aid. or at least not hinder, burrowing life. 
As compensation for loss of limbs, they have developed, as have nearly all 
other groups so handicapped, a greatly lengthened spine. We have 33 vertebrae 
or, allowing for those parts of the spine where several vertebrae have fused, 
26 separate bones in our spine. Snakes have up to 400. This restores mobility to 
the degree that Mambas and egg-eating snakes can travel freely through the 
branches of trees. As a contribution to slimness of body, the left lung has been 
reduced in size and in many species been entirely suppressed. The kidneys lie one 
in front of the other, instead of being side by side as in quadrupeds. Then, as 
an additional aid, they have been helped by development of a row of hinged, 
backward-pointing scales on the ventral side of the body. These give an additional 
grip on whatever surface the snake is travelling over, and probably account for 
the superior mobility of the Adder, say, over the Slow-worm. I once saw a 
Slow-worm trapped in a shallow, smooth pot drainage trough in a garden. It was 
quite able to escape when lifted on to level, rough ground; but I feel sure an 
Adder or Grass-snake could have got out unaided. 
The next most important mutation which has aided the Snakes, is the 
loosening of the jaws. The two halves of the lower jaw are joined by an elastic 
ligament, and the joints onto the skull, at the back of the jaws, allow the mouth 
to open so widely that most snakes can swallow prey larger than the unstretched 
circumference of their body. Snakes have in many cases fairly good eyesight, good 
enough in the spitting snakes (the Spitting Cobra and one or two others) to aim 
at an aggressor's eyes. Though hearing is apparently lost, sense of smell is acute, 
and the flickering tongue carries scented particles into the mouth, where they are 
tasted in Jacobson's organ, a sensory pit in the roof of the mouth. Rattlesnakes 
and Pit-vipers have on the front of the head a pair of organs which are not 
paralleled, so far as I know, by any other vertebrates, pits sensitive to infra-red 
radiations, enabling their owners to detect warm animals in the dark. Of course 
the principle here is analogous to a lady holding a flat-iron a few inches from 
the cheek to test its heat, but in degree it is in quite another class. 
Some snakes, like Grass-snakes, merely seize their prey as any lizard might 
do and, aided by backward curving teeth, swallow it whole and trust to asphyxia 
and digestive juices to kill it in the stomach. But others, the Boas and Pythons, 
make use of their length and the mechanical advantage of their form of muscula- 
ture to compress their prey so that it dies from inability to draw breath and be 
swallowed at leisure. 
However, the real speciality which accounts for most of the wide distribution 
of snakes is a series of mutations which have turned certain teeth into syringes 
and salivary glands into producers of deadly poisons. In some snakes, several back 
teeth serve as fangs; a pair of front teeth, a more efficient arrangement, are 
developed in Sea-snakes and Cobras, and in a third group, Vipers and Rattle- 
snakes, is the most specialised arrangement. A pair of front teeth serve as fangs 
and are hinged so that they lie flat along the jaw when the mouth is closed, but 
are erected when the mouth is opened wide to strike. The actual venom appears, 
as I have said, to be modified saliva and the poisonous chemicals are classified as : 
1. Neurotoxins, which paralyse. 
2. Haemorrhagins which destroy the lining of blood vessels and cause bleeding. 
3. Anticoagulants. 
4. Thrombase which causes b^ood to clot. 
5. Haemolysins and cytolysins which destroy living cells in the blood and 
body tissues. 
6. Digestive enzymes, probably not clearly distinguishable from the last group. 
Different species of snakes have varying proportions of several of these, so 
that snake-venoms tend to be very complicated mixtures of chemicals — Cobras 
kill mainly with Neurotoxins, producing paralysis; Vipers and Rattlesnakes by 
Haemorrhage and Tissue Destruction, but Russell's Viper venom contains so much 
blood-clotting factor that it is used as a life-saving remedy in cases of Haemo- 
philia, in which disease natural clotting of blood in wounds is defective. 
