So much for the size of rattles. Now for the development of them.
The theory that the rattle is the remains of cast-off cuticle, as some herpetologists have supposed, may be dismissed at once; for what would cause such vestiges to harden into a complicated and symmetrical form?
To Dumeril we owe some of our best conceptions of the growth of the rattle, which, whether it has or has not been evolved from the mere horny spine that terminates the tails of so many snakes, has certainly now an express provision for its production.
Like hair, claws, or nails, the rattle is horny matter excreted and hardened. In his Elementary Lessons in Physiology, Prof. Huxley shows us how in the growth of a nail new epidermic cells are added to the base, constraining it to move forward. ‘The nail, thus constantly receiving additions from below and from behind, slides forward over its bed and projects beyond the end of the finger.’ If the reader will look at his finger nail, and suppose the end bone of the Crotalus spine to be the ‘bed’ of the nail, he will to a certain extent be able to comprehend how the rattle grows out; but that the links become detached in succession is a phenomenon so astonishing and at the same time so difficult to comprehend, that few naturalists have ventured to state positively how this occurs. Conjecturally only and diffidently do I, therefore, presume to offer a supposition; and if my readers will once more pardon reference to human nails, and lend the aid of their imagination, they may be able to evolve a true theory out of my crude idea.
The young readers of Aunt Judy’s Magazine were also, a few years ago,[84] invited to lend the aid of their pink little finger nails to the illustrative development of a supposed rattle; and we will again imagine the whole tip of a finger to be covered with a round nail-cap, proceeding from the first joint, and to have grown so from birth. In growing out, this curious, cup-like nail, being never cut, would become hollow like a thimble. Pointed or tapering it would of course be, because, as the baby finger grew, the base or new portion of nail grew larger with it. We will also suppose that the joint whence the nail sprang was in constant activity, and so articulated that it could move with a quick and regular action or vibration; the hollow nail-cap, having attained a certain size, would become withered, and (as the constant bending of a piece of card or metal in time divides it) would be worn, and at length detached at its base. Meanwhile the growth of nail has not been arrested, but a new cap is forming within. The old, dry, and withered cap has now nothing to retain it, and would drop off, on account of its simple, conical form, like a loose-fitting thimble. But Dumeril explains to us that the terminal bones of the rattlesnake’s spine present a peculiar form, several of them coalescing.
‘Dans les Crotales cette extremité de la queue, au lieu d’être pointue, se trouve comme tronquée, et, par une bizarrerie que nous n’expliquons pas, il paraîtrait que les trois dernières pièces de la colonne vertébrale se seraient soudées entre elles, et comme aplaties pour composer un seul os triangulaire, avec trois bourrelets latéraux simulant des restes d’apophyses transverses des vertèbres, ainsi qu’on les voir souvent dans les trois dernières pièces du coccyx chez l’homme. Cet os anormale a été disséqué chez un Crotale, on a reconnu qu’il est recouvert d’une sorte de matière cartilagineuse dans laquelle aurait été secretée la substance cornée, comme un epiderme solide, qui conserve en effet extérieurement la forme de la pièce osseuse, sur laquelle elle a été en quelque sorte moulée et qu’elle semble destinée ainsi à protéger contre l’exfoliation, comme cela s’observe dans ceux des animaux ruminants dont la corne revêt les chevilles osseuse du véritable coronal prolongé en pointe et devenu de cette façon une arme d’attaque, et surtout de defence.’[85]
Dumeril also tells us that the peculiar structure of those few terminal vertebræ, with their knobs or pads (‘bourrelets’) upon which the skin is moulded, tends to a movement lateral rather than up and down,—that quick action which we perceive when the rattle is being vibrated. Thus the horny covering takes the form of this bone with its lobes or bulges, which instead of permitting the supposed cup-like nail to fall off as in our finger illustration, causes the links as they are pushed out to hang or cling together; and we can only suppose that the constant action loosens, and not only loosens when dead or detached, but loosens, that is to say, enlarges, the link while growing. For if you examine the spine of a skeleton Crotalus and the rattle that grew upon that spine, you will perceive that the links are a great deal larger than the ‘pièce osseuse sur laquelle elle a été en quelque sorte moulée.’
There is one other peculiarity observable in a detached rattle, which I cannot pretend to explain in any way. If you hold one up by its base or largest link, you will find it invariably hangs in a slight curve and not perpendicularly. You can straighten it, but you will not be able to curve it in the opposite direction, proving that it naturally inclines one way, whether to the right or the left of the animal while living, I cannot assert. But it is a curious feature, and one that can no doubt be accounted for by scientific observers. Thus, as in the illustration below, you can curve a rattle so as to discern the interior links on one side, but not on the other. I have made the attempt with many rattles, but always with the same result. The centre fig. below is a section.
Natural position when held.————Straightened by force.