These alternate inspirations and expirations, with their accompanying movements, the swelling and then diminishing of the trunk and the regular _soufflants_, are so precisely like those of a pair of bellows, that excepting in shape, we require no more complete comparison. The _degree_ or strength of hiss is in this reptile very perceptible. When recently imported and easily excited, its violent ‘puffing’ corresponds with a very large pair of bellows; but in time it grows less alarmed at the appearance of the human beings who unceremoniously stare at it; and at length the puffing is very slight, ceasing altogether after the snake becomes accustomed to its surroundings. But if molested and alarmed, you then see the full play of the lungs, and the whole body alternately expanding and contracting as before. We may almost compare this pulmonary action to the panting or full breathings of ourselves under alarm or agitation. Only, in comparison as the lung of snakes is elongated, and there is so much of it to fill with air, so is the sound prolonged, and the breathing a slower process. There is another viper, the small Cape adder (_Vipera atropos_), a most deadly little reptile, in which a similar sound to that of the ‘puff adder’ may be heard. When this creature is disturbed, it draws in a long breath which expands its whole body in the same manner, and then in expelling the air, a long sort of wheeze or blowing is audible. Even in drawing the breath in, a slight sound is heard (as it also is in our native viper and some others); but instead of the prolonged hiss by which most snakes display their agitation, this little adder expresses itself in long successive blowings, like its larger relative _arietans_, only a little less regularly. In the present instance, I saw the lung inflated with an agitated undulating motion, as if the fluid air were entering in little waves. I do not state positively that this is invariably the case from having witnessed it in one specimen. This might be the normal process, or this viper’s lungs and health may have been impaired. I am thus precise because it is unsafe to establish as an invariable fact in natural history what may have been seen only occasionally, a habit which has so often led to the promulgation of erroneous impressions. The prolonged sound of the hiss in snakes is due to the size of the lung, they having a large supply of air to draw upon. Some serpents expand their bodies under excitement without any perceptible hiss: the cobra both hisses and expands, so do some others; but all these movements are, no doubt, connected with respiration in some way, just as in human beings, sighing, sobbing, panting, etc., in which the ribs take part, are only modifications of the ordinary movements of respiration, and chiefly emotional.