XII. Observations on the Neck of the Three-toed Sloth, Bradypus tridactylus, Linn. By THOMAS BELL, Esq., F.R.S., L.S., G.S., 8 Z.S.

Communicated August 13, 1833.

THE laws which regulate the numerical variations in the different systems of organs in animals, are perhaps less defined, or at least less understood, than those which relate to many other conditions of their existence. In some cases, indeed, these variations appear to be wholly anomalous ; but in others the normal number of parts is so strictly adhered to, as to be absolutely without any known exception in a whole group.

Amongst these, one of the most obvious and remarkable is the restriction of the cervical uertebrcc in the whole of the class of mammiferous animals, to the number seven. That this number should be found equally in the short interval between the cranium and the thorax, scarcely deserving the name of a neck, which we see in the Cetacea, and in the long flexile neck of the Camel and the Cirafe, is indeed a striking and in- teresting fact, and may be viewed as an important illustration of that law which pro- vides for the most considerable variations in the offices or functions of a part, rather by a modification, in form or size, or even situation, of organs already existing and es- sential to the type of the group, than by the production of new organs on the one hand, or, on the other, by the abstraction of any which appertain to the normal form.

To this normal number, however, the Ai, Bradypus tridactylus, Linn., has for many years been considered as an exception ; as by the examination of numerous specimens; the neck was found to possess nine vertebre, which were all believed to belong to the cervical class.

An isolated exception to a rule so general, and obtaining in cases of such diversified forms as those to which I have alluded, presents itself to the mind of every one accus- tomed to look at the general harmony of the established laws of formation, as a vio- lation of that unity of design which constitutes one of the most interesting objects of our investigation, especially as the exception itself is abrupt and sudden, and without any of those intermediate gradations of structure by which the mind is prepared, as it were, for considerable diversities of form, and which so generally soften the transitions which the different offices of the same organ in different groups may render necessary. It was from this consideration, rather than as merely correcting a generally received error, that I found, with feelings of no ordinary satisfaction, that in truth this nume- rical law is not departed from in the present instance, and that the animal in question forms no such exception to the general rule as had been asserted ; the two vertebre which have hitherto been considered as the eighth and ninth cervical, being in fact the

114 MR. T. BELL'S OBSERVATIONS ON THE NECK

first and second dorsal, each of them bearing a pair of rudimentary ribs, moveabIy arti- culated to their transverse processes by a true articular surface. This fact I have ascer- tained by the examination of two skeletons in my possession, one of which is an adult, and is artificially articulated, the other very young, and preserved as a natural skeleton in spirit.

In the adult animal we find the eighth and ninth vertebre, which I shall now call the first and second dorsal, having the transverse processes longer and narrower than those of the cervical, and each terminated with a perfect articular surface, which is slightly depressed ; and to these are attached the heads of the rudimentary ribs just mentioned. The first of these rudiments is small and slender, about ;,ths of an inch in length, having a distinct rounded head at the articular extremity, becoming then abruptly smaller, and tapering to the apex. The second is considerably larger, and assumes more of the character of a short rib. It is about 6 lines in length, and nearly 2 in breadth. Its head is oblong and rounded, and there is a tubercle on the upper and anterior side. Towards the extremity it becomes broader and flatter, with an excavated surface inwards, and a convex rough prominence on the outer side, apparently the point of muscular attachment. Immediately behind and beneath the head of the bone is a minuteforamen for the passage of intercostal vessels.

The character of the transverse processes of these two vertebre differs very materially, as might be expected, from that of the true cervical. In the superior vwtebre this process is transverse and slightly hifid. In the seventh cervical it stands obliquely forwards, and the apex is broad and oblong. In the first dorsal each transverse process is completely divided into ah anterior flattened process, which is turned forwards, and a true lateral or transverse one, which supports the little rudimentary rib., The trans- verse process is smaller, but considerably longer than those of the true cervical, and stands more in a lateral or transverse direction. In the second dorsal vwtebra the an- terior processes do not exist, and the body assumes the form of the succeeding ones. The transverse processes 'are simple and obtuse, and the articular surface is slightly excavated.

In the natural skeleton to which I have referred, the rudimentary ribs are very ob- vious, though, from the early age of the subject, they are of course much smaller than in the former. The first, indeed, consists only of a minute particle of bone, not much larger than a pin's head, but connected with the vertebra by a capsular ligament, and perfectly moveable ; the second is of more considerable size, and, like the former, has its capsular ligament inclosing its head, and holding it on to the articular cavity of the transverse process of the vertebra.

Cuvier appears to have seen the moveable costal rudiment in the young animal ; he has, however, evidently confounded it with the long transverse process in the adult, and has wholly passed over the obvious analogy which I have here endeavoured to trace. He says, " Les apophyses transverses du cou sont courtes, larges au bout, qui

OF TIIE THREE-TOED SLOTH. 115

est oblique, se baissant un peu en avant, et y rentrant un peu en dedans. La huitihme a la sieniie un peu fourchue. La neuviZme l’a prolonge‘e en une petite pointe qui se porte en avant et en dehors. Dans le jeune individu cette partie n’est pas soudde ti la vertZbre ; seroit-ce un petit vestige de c6te 1

In the second edition of the ‘Rhgne Animal’ occurs the following observation, showing what were the latest views of Cuvier on this subject. “ C’est le seul mam- mifhre connu jusqu’ti ce jour ( p i ait neuf verthbres cervicales.”z

The nearest approach which has hitherto been made to the true bearing of the fact, is contained in the following passage from Meckel. Speaking of the points of ossifi- cation or nuclei in the cervical vertebre, he says, “ In the last are found a fourth and a fifth [nucleus], constituting, as it were, rudiments of ribs, projecting from the sides. In Man this elongated bone forms the anterior root of the transverse process, and extends from the body to the posterior root of that process. In the AX a very considerable bony nucleus is articulated by means of a broad cartilage to the end of the transverse process of the ninth cervical certebra ; by means of which this vertebra becomes suddenly much broader than the rest. In the other Mammalia which I have examined, this bony nu- cleus is wanting. I t is remarkable that in the Ai’ an analogous but much smaller bony nucleus is found attached to the same situation in the eighth cervical vertebra ; so far as this goes, these two vertebra become similar to dorsal, and thereby the exception which the Ai’ makes in this particular to other Mammalia is lessened.”3

From this passage it is evident that Meckel still considered the two cer t ebr~ in question to be truly cervical, though approaching to the character of dorsal vertebra.. The fact, however, that the rudimentary ribs remain permanently moveable, of which it would appear that both Cuvier and Meckel were ignorant, at once proves that these vertebre are not only approaching to the dorsal form, but are essentially dorsal, if it be a true character of a rib, as distinguished from a transverse process, that it is perma- nently moveable. This is a question certainly of considerable interest, but one into which it is not necessary on the present occasion to enter, as the fact of the permanent mobility of the rudimentary ribs in the Ai, the perfect construction of the capsular ligament, and the cartilaginous surfaces of the joint are sufficient, joined with the ex- istence of a foramen for the passage of vessels and other circumstances in their struc- ture and situation, to establish their character beyond all doubt. The rule therefore which assigns seven cervical vertebra to the whole of the Mamnmlia, is thus left with- out a single exception.

The interesting paper of Professor Buckland on the habits of the Sloth, lately read at the Linnean Society, precludes the necessity of my entering into any lengthened specu- lations on the utility of this singular structure ; I may, however, remark, that the fact

1 Ossemens Foss., tom. v. p. 63. 3 Syst. der Vcrgleich. Anat., B. ii. 111. 2. p. 294,

* RBgne Anim., (ed. 2.) tom. i. p. 252.

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116 MR. T. BELL ON THE NECK OF THE THREE-TOED SLOTH.

that these vertebrQ are dorsal instead of cervical, does not in the least affect the question of their office. The object of the increased number of vertebre in the neck is evidently to allow of a more extensive rotation of' the head ; for as each of the bones turns to a small extent upon the succeeding one, i t is clear that the degree of rotation of the ex- treme point will be in proportion to the number of moveable pieces in the whde series. When the habits of this extraordinary animal are considered, hanging, as it does, sus- pended from the under surface of boughs with the back downwards, it is obvious that the only means by which it could look down towards the ground must be by rotation of the neck ; and as it was necessary, in order to effect this without diminishing the firmness of the cervical portion of the vertebral column, to add certain moveable points to the number possessed by the rest of the class, the necessary additional motion was acquired by modifying the two superior dorsal vertebre, and giving them the office of cervical, rather than by infringing a rule which is thus preserved entire, without a single known exception.

PLATE XVII.

Fig. 1. The two last cervical and four first dorsal vertebra of Bradyps tridactylw.

Figg. 2 and 3. The two rudimentary ribs enlarged to three times their natural mag- a. the first rudimentary rib ; b. the second rudimentary rib.

nitude.