The length of it naturally varies according to the size and species of serpent; but as a rule it is always much longer comparatively than in man. In a full-sized rattlesnake, the trachea is about twenty inches long. In a boa constrictor, also, though a much larger snake, it measures about the same. In smaller snakes it is, of course, much shorter; but there is the same singular diversity in this as we find in other serpent anomalies, viz. a great variation in the length in snakes of equal size, and without any very apparent reason.
Bingley, in his Animal Biography, 1820, describes the appearance of a large snake (M’Leod’s celebrated boa) when gorging a goat; but the account, like those of that time, is more sensational than scientific. ‘His cheeks were immensely dilated, and appeared to be bursting, and his windpipe projected three inches beyond his jaws.’
Broderip, a few years later, 1825, more lucidly and dispassionately describes what he had observed. ‘I have uniformly found that the larynx is, during the operation of swallowing, protruded sometimes as much as a quarter of an inch beyond the edge of the dilated lower jaw. I have seen, in company with others, the valves of the glottis open and shut, and the dead rabbit’s fur immediately before the aperture stirred, apparently by the serpent’s breath, when his jaws and throat were stiff, and stretched to excess’ (Zoological Journal, ii. 1826). This account is quoted from the paper entitled, ‘Some Account of the Mode in which the Boa Constrictor takes its Prey, and of the Adaptation of its Organization to its Habits,’ by W. J. Broderip, Esq., F.L.S. The paper was written as a criticism of the M’Leod story.
I, also, on several occasions, saw the fur or feathers stirred by air when the mouth or valve opened of what we may safely call the air-tube, whether larynx or trachea.
Though so rarely mentioned in popular books on snakes, this surprising modification of the breathing apparatus was described by the indefatigable Dr. Edward Tyson, on his dissection of the first rattlesnake that fell into the hands of the Royal Society, 1683, and whose paper on the Vipera caudisona, as he named it, is quoted in chapters xvi. and xx. ‘Over the tongue did lye the larynx, not formed with that variety of cartilages as is usual in other animals, but so as to make a rime or slit for receiving or conveying out the air. Nor was there any epiglottis for preventing other bodies from slipping in, this being sufficiently provided for by the strict closure of it.’[38]
Dr. Tyson examined only a dead specimen, and could not therefore witness the action observable in life; but his remarkable accuracy in describing the parts will be evident in comparing what he said with Dumeril, who did observe the living reptiles. The confusion which sometimes occurs in distinguishing the parts may be also explained by the less complicated structure of the tube, which in higher animals presents the nicer distinctions of the parts, glottis, epiglottis, larynx, etc.
‘Il n’y a pas de véritable larynx, une petite languette mobile qui s’ajuste, sur l’ouverture linéaire; c’est la glotte.... La glotte, située au-dessous de la victime, se porte en avant, et l’acte de respiration ne se trouve point empêché. C’est que nous avons indiqué à l’article de la déglutition; car on voit distinctement alors la glotte se fermer et se délater.’[39]
This petite languette became a new object of curiosity, and soon came fresh opportunities for observation, namely, when some of the larger snakes were engaged in yawning. On account of its extreme mobility, you do not always detect the form of this little point on the upper lip, which as often as not presents a rounded opening; but occasionally the little tongue—which can be nothing but an apology for an epiglottis—is very distinct, and may be compared with the moveable, pointed snout of some of the large pachyderms, or, still better, with an exactly similar formation at the end of the elephant’s trunk, and which, though for a different purpose, moves similarly.
As to the exact position of this glottis when at rest, a word or two must be said; for a number of prepositions have been used to describe it. One writer says ‘beneath’ the tongue sheath, others say ‘beyond,’ others again ‘before;’ ‘over,’ ‘above,’ ‘behind,’ ‘in front of,’ have been variously used, and all depending on which way the snake is viewed; but without drawing upon half a score of prepositions to puzzle the reader, as I myself was sorely puzzled until a yawning snake was so kind as to afford me an ocular scrutiny of its lingual arrangements, we can easily comprehend where a passage to the windpipe and lungs must necessarily be, and which, it is clear, is not under the tongue. When a snake’s head is raised, as in crawling up a wall or a tree, the glottis may be said to be ‘beneath’ or ‘under;’ but the general position of a snake being horizontal, the mouth then opened would show you the opening of the tongue sheath nearest to you and to the front; and beyond that, behind, over, or upon the tongue sheath, is another aperture, which is the glottis or entrance to the larynx and trachea or windpipe.
So there are in fact two sheaths or tubes lying one upon the other, viz. the tongue sheath, and upon this and parallel with it, the windpipe.