Experiments made with the secretion of the parotid gland of _Tropidonotus_ and _Zamenis_ have shown that even Aglyphous snakes are not entirely devoid of venom, and point to the conclusion that the physiological difference between so-called harmless and poisonous snakes is only one of degree, just as there are various steps in the transformation of an ordinary parotid gland into a poison gland or of a solid tooth into a tubular fang. The question whether all snakes are immune to their own poison is not yet definitely settled. Most snakes certainly are, and it is a remarkable fact that certain harmless species, such as the North American _Coronella getula_ and the Brazilian _Rhachidelus brazili_, are proof against the poison of the Crotalines which frequent the same districts, and which they are able to overpower and feed upon. The Cribo, _Spilotes variabilis_, is the enemy of the Fer-de-lance in St. Lucia, and it is said that in their encounters the Cribo is invariably the victor. Repeated experiments have shown our Common Snake, _Tropidonotus natrix_, not to be affected by the bite of _Vipera berus_ and _V. aspis_, this being due to the presence, in the blood of the harmless snake, of toxic principles secreted by the parotid and labial glands, and analogous to those of the venom of these Vipers. The Hedgehog, the Mungoose, the Secretary Bird, and a few other birds feeding on snakes, are known to be immune to an ordinary dose of snake poison; whether the pig may be considered so is still uncertain, although it is well known that, owing to its subcutaneous layer of fat, it is often bitten with impunity. The Garden Dormouse (_Myoxus quercinus_) has recently been added to the list of animals refractory to Viper poison. ------------------------------------------------------------------------ CHAPTER VII NERVOUS SYSTEM--SENSE ORGANS The brain is small and of very oblong shape. It consists of smooth cerebral hemispheres, small optic lobes, a still smaller cerebellum, and long olfactory lobes; the pineal body is not accompanied by a parietal organ. The spinal accessory cranial nerve is absent, and the sympathetic system is but feebly developed. The eyes have been noticed above (p. 12). When normally developed they are susceptible of a slight movement under the transparent disc, quite independent from the cornea, which covers them, and from which they are separated by the so-called "lacrymal chamber." There are two lacrymal glands, one in front and one behind; the lacrymal duct opens into the posterior nares. A sclerotic bony ring is absent.