This no doubt answers to the ordinary ‘hissing’ of the majority of snakes; but that the sound varies under certain conditions, and in the same serpent, cannot be denied. A. R. Wallace relates an incident which may well be introduced here, as affording both a proof of the length of time snakes can sustain a sort of half suffocation, and also the expression or power of ‘voice’ in breathing. A young boa was caught, and in order to prevent its escape, its captors, while preparing a box in which to convey it away, tied it tightly round the neck to a thick stick, which not only fettered its movements, but appeared to nearly stop its respiration. It lay writhing in much discomfort, sometimes opening its mouth with a suspicious yawn, as if trying hard to breathe. By and by, when relieved from its clog and safely consigned to a box with bars on the top, it began to make up for loss of time by breathing violently, ‘the expirations sounding like high-pressure steam escaping from a locomotive. This continued for some hours, of four and a half respirations a minute,’ when the breathing—in this case we may say panting—gradually subsided, and then the poor thing settled down into silence.[46] The expression of feelings by the tail in so many snakes, producing a sibilant sound in rustling dead leaves, and in some which are supposed never to hiss, is a subject well worth the attention of scientific naturalists. It would be interesting to ascertain if any peculiarity of trachea or of glottis exist in these. [Illustration] [Illustration] CHAPTER X. _HIBERNATION._ THE periodical torpor known as the winter sleep of reptiles is intimately connected with respiration, and a chapter must now be devoted to this subject. ‘Reptiles are obedient to the external atmosphere,’ has been aptly said of them. Thus, they obey the sun; for if exposed to his rays, they warm into life and activity. They obey the frost; for when exposed to its influence, their functions grow feeble or fail altogether, and they succumb to within a verge of lifelessness. They obey all the intermediate variations of temperature during the changing year, by displaying degrees of animation and activity responsive to the degree of warmth externally which they do not possess in themselves.