But let us turn to the hard, cold, unpoetical, unimaginative language of science, and see what a snake can really do in the vocal expression of its feelings. Says Dr. Carpenter: ‘In all air-breathing vertebrata the production of sound depends upon the passage of air through a certain portion of the respiratory tube, which is so constructed as to set the air in vibration. In reptiles and mammals it is at the point where the windpipe opens into the front of the pharynx, that this vibrating apparatus is situated. Few of the animals of the former class, however, can produce any other sound than a _hiss_, occasioned by the passage of air through the narrow chink by which the trachea communicates with the pharynx; but this sound, owing to the great capacity of their lungs, is often very much prolonged’ (_Animal Physiology_),—prolonged, but not powerful, be it observed. Says Professor Owen: ‘The true “_chordæ vocales_” are absent in serpents, and the voice is reduced to a hissing sound, produced by the action of the expired air upon the margins of the glottis’ (_Anatomy of the Vertebrates_). Speaking of the escape of air from the lungs, Dumeril says: ‘Lorsqu’il est passé plus vivement il laisse entendre une sorte de vibration, qui le plus souvent, ne consiste que dans le bruit d’un soufflement.’[40] Sometimes, according to the position of a snake, or when the passage is well open and uninterrupted, the hiss partakes somewhat of a whistling sound, like the blowing through a quill. I observed this particularly in a ‘tree boa’ (_Epicratis cenchris_), which hissed at me angrily one day because I took the liberty of touching it when the keeper opened its cage to arrange its blanket. The ‘hiss,’ not loud, or by any means musical, differed from the ordinary blowing only as a current of air passing through a round tube would differ from the same current passing through a narrow slit. A true ‘hiss,’ such as we produce with closed teeth in prolonging the sound of _s_, a serpent can never express. The nearest approach to it in the human voice is when the tongue is in the position as if we are about to say _ye_ or _he_, and then prolong the breath; that is to say, breathe out while the tongue is so placed before the word is uttered. Naturally the larger the snake the stronger the ‘hiss;’ the more rapid the expiration, the more powerful will be the volume of air with its attendant _soufflement_. The sound and action, as well as degree, are easily seen in the ‘puff adder’ (_Clotho_, or _Vipera arietans_). When angry or alarmed, it draws in a full breath, and its body swells perceptibly; then you hear the escaping air like a prolonged sigh or blowing till the lungs are empty. This process is repeated as long as the provocation lasts.