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An Account of a Peculiarity not hitherto described in the Ankle,
or Hock-joint of the Horse ; with Remarks on the Structure of the Vertebræ in the Species of Whale, entitled Delphinus Diodon * By ROBERT J. GRAVES, M. D., M. R. I. A., King's Professor of the Institutes of Medicine, Honorary Member of the Royal Medical Society of Berlin, of the Medical Association of Hamburgh, &c. &c.
Being engaged in the dissection of the horse, on examining the hock-joint, I found that any effort to flex or bend the limb at that joint, was counteracted by a considerable resistance, which continued until the limb was bent to a certain extent ; after wbich, suddenly and without the aid of any external force, it attained to its extreme degree of flexion. In attempting to restore the extended position of the limb, I found that a similar impediment existed to its extension, until the same point was passed, when the limb suddenly, as it were, snapped into its extreme degree of extension at this joint.
At first I conceived that this phenomenon depended on the tendons of the flexor and extensor muscles of this joint; but on removing all these muscles and their tendons, it was not diminished, and it therefore became clear that it depended on some peculiar mechanism within the joint itself.
Before I enter into the details of this mechanism, it is necessary to remark, that it is evidently connected with the power this animal possesses, of sleeping standing, for it serves the purpose of keeping the hock-joint in the extended position, so far as to counteract the oscillations of the body, without the aid of mus. cular exertion ; and in this respect it resembles the provision made to effect a similar purpose in certain birds, as the stork, and some others of the grallæ, which sleep standing on one foot. It will appear, also, in the sequel, that not only is the effect produced the same, but the mechanism is in many respects similar, if the account given by Cuvier, and also by Dr Macartney, in Rees' Cyclopædia, article Birds, be correct.
• Read 5th July 1830, before Royal Irish Academy, and just published in the Memoirs.
Sheep and cows are not provided with ankle-joints of a similar structure, and it is well known that these animals do not possess the power of sleeping standing. Another circumstance which adds additional interest to this peculiarity of structure, is, that it may possibly be connected with the disease termed String-halt, in which the limb is at each step suddenly flexed, to a degree far beyond that required in ordinary progression. Whether this is owing to a sudden and jerking flexion of the whole limb, or to flexion of the hock-joint alone, I have had no opportunity lately of determining. If the latter be the case, it is probably connected with the structure of the hock-joint, which I am about to describe. It may be right to observe, that not even a probable conjecture has been advanced, concerning the nature and cause of string. halt, a disease to which the sheep and cow are not subject, and we have already observed, that in these animals the structure of this joint presents nothing remarkable.
The hock-joint is a good example of what is termed the hinge-like articulation, and is formed between the tibia and astragalus, which latter bone presents an articulating surface, with a nearly semicircular outline, and divided into two ridges, including between them a deep fossa. The tibia is furnished with depressions which ride upon the ridges of the astragalus, and has anterior and posterior projections, which, moving in the fossa, are received into corresponding depressions in the astragalus, at the moment the limb arrives at the greatest degree either of flexion or of extension.
The shape of the surfaces of the astragalus concerned in the articulation, is not that of a given circle throughout, for towards either extremity, the descent is more rapid, or, in other words, answers to an arc of a smaller circle. Hence, when one of the projections of the tibia has arrived at its corresponding cavity in the astragalus, which happens when the limb is either completely flexed or completely extended, the rapid curve of the articulating surface presents a considerable obstruction to change of position. Thus, the form of the articulating surfaces, in itself, to a certain degree, explains the phenomenon ; but its chief cause is to be found in the disposition and arrangement of the ligaments.
The external malleolus of the tibia is divided by a deep groove, for the passage of a tendon, into an anterior and posterior tubercle ; from the latter of which, and close to the edge of the articulating surface, arises a strong and broad ligament, that is inserted into the os calcis. Under this lies another ligament, which, arising from the anterior tubercle, is also inserted into the os calcis. It is to be observed, that the origin of the latter is anterior to that of the former, but its insertion posterior, so that these lateral ligaments cross each other in the form of an x. The external articulating protuberance of the astragalus on which the tibia revolves, has, as has been already stated, a nearly circular outline, and the attachments of the ligaments just described, are at points on the outside of the os calcis, which would lie nearly in the circumference of that circle, were it continued from the articulating surface; so that each of these ligaments has one of its extremities fixed in a certain point of the circumference, while its opposite extremity revolves during the motion of the joint, nearly in the circumference of the same circle. This observation applies likewise to the two lateral ligaments on the inner side of the joint, which have nearly the same relation to each other, and to the general contour of the joint, as that just described ; so it is obvious, that during the rotation of the joint, as the origins of these ligaments move along the same circumference in which their attachments are fixed, the ligaments will be most stretched when they correspond to diameters of that circle.
Now it is so arranged that this happens at the same time for all, and consequently the ligaments on each side correspond not merely as to direction, but as to the point of time they become most stretched, which is nearly at the moment that the joint has no tendency to move either way, and at that moment, it is to be observed, that although the ligaments are most tense, and of course react on their points of attachment with greatest force, yet this produces no motion, as the force is exerted in a direction perpendicular to the circumference; but as soon as the tibia is moved beyond this point of inaction for the ligaments, the latter, no longer representing diameters, by their contractile force evidently tend to accelerate the motion; and as they all act in the same direction, and are assisted by the shape of the arti. culating surfaces, a sudden motion of flexion or extension is thus produced.
The preceding explanation supposes the ligaments of this joint to possess, contrary to the nature of ligaments in general, a certain degree of elasticity, which was evidently the case in all, but particularly in the most deep-seated of those on the inner side of the joint, which, therefore, appears most concerned in producing the sudden motion, whether of flexion or extension.
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In the autumn of 1829, two of the species of whale called Delphinus diodon, by Hunter, Hyperoodon, by La-Cepede, and Cetodiodon, by Dr Jacob, were captured near Dublin, one of which, measuring about sixteen feet in length, I procured for the purpose of preparing its skeleton.
After the spinal column had undergone maceration for a few days, I found that the intervertebral substance could be easily detached from the bodies of the vertebræ, and that it carried with it, firmly attached to each of its extremities, a flat circular bone; about a quarter of an inch in thickness, and exactly corresponding in the extent and shape of its surface, to the surface of the body of the vertebra, from which it had been separated.
The separation was effected with facility, and took place spontaneously and completely when the maceration had been continued some time longer.
The surface of the flat bone, where it had been adherent to the body of the vertebra, was of a spongy texture, afforded a passage to many bloodvessels, and was marked by numerous sharp projections and deep furrows, diverging from its centre, and answering to similar projections and furrows on the denuded extremity of the vertebra ; of course the surface of these bones varied in shape and size with the extremities of the vertebræ to which they were attached, being from five to six inches in dia. meter at the dorsal, and not more than one inch at the last caudal vertebra.
The substance of these bones towards the intervertebral substance was of much harder and closer texture than that of the bodies of the vertebræ themselves, and where it was adherent to
the intervertebral substance, it had a smooth surface, marked with a great number of concentric lines, answering to the arrangement of the fibres in the intervertebral tissue, which adhered to this face of the bone with great strength. This marking was deficient towards the centre where the intervertebral substance is fluid.
The facility with which these bones are detached, is the reason why we never find them adhering to the vertebræ of those young whales which have been wrecked on our coast, and whose skeletons have been exposed to the action of the waves and the weather. Their flat shape, too, renders them liable to be covered by the sand, and hence I have never known them to be found separately, even when the vertebræ and other bones of this species of whale were scattered along the coast in great numbers, as happened at Dungarvan some years after several of these animals had been captured and dragged ashore by the fishermen *
The bones I have described must evidently be considered in the light of terminal epiphyses of the bodies of the vertebræ, and are deserving of notice on account of the facility with which they can be detached, even in very large, and of course not very young, animals of this species, as I observed in the two skeletons preserved in the College of Surgeons, one of which measures thirty feet in length; so that when the skeleton has been artificially prepared, they resemble separate intervertebral bones rather than vertebral epiphyses. In the land mammalia the consolidation takes place much more rapidly, and a few years are sufficient to efface all traces of former - separation between the epiphysis and the body of the vertebra; the comparative slowness of this process in the whale, is probably referrible to the longevity of the animal, and the greater length of time necessary to complete its growth. A knowledge of this fact puts us in possession of a new and useful mark of the animal's age, independent of its size, and it is for this purpose I have brought it forward, for although not noticed by any author I have seen
• Many years ago we picked up several of these intervertebral looking bones, upwards of a foot in diameter, on the shore of the island of Yell, one of the Shetlands. In their neighbourhood was a skeleton of a whale, about 40 feet long, part of which we brought to Leith.--EDIT..