272 



THE CABINET OF NATURAL HISTORY 



rors. Theodosius gave fights of animals in the circus; and 

 Justinian himself exhited in the amphitheatre twenty lions 

 and thirty panthers. 



Such sights, repeated without interruption for more than 

 four hundred years, must have afforded the Roman naturalists 

 opportunities of making numerous observations on the 

 forms, habits, and interior organization of foreign animals; 

 yet science was little improved by their labours. It seems, 

 that the animals being once killed, nobody derived any 

 further benefit from their slaughter. The proof is, that 

 all the writers of the first, second, and third centuries of 

 the Christian era, who have treated of such animals, have 

 borrowed every thing they have said about them from 

 Greek authors who lived before the Roman conquest. Pliny 

 himself is but a compiler. — From a Lecture delivered by 

 Baron Cuvier. — JEdin. Phil. Jour. 



A PECULIAEITY NOT HITHERTO DESCRIBED 



IN THE ANKLE OR HOCK-JOINT OF THE HORSE. 



By Robert J. Graves, M.D., M.R.LA. 



Being engaged in the dissection of the horse, on exam- 

 ining 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 which, 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. 



x\t 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 de- 

 pended on some peculiar mechanism within the joint it- 

 self. 



Before I enter into the details of this mechanism, it is ne- 

 cessary to remark, that it is evidently connected ivith 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 muscular 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 grallse, 

 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 ac- 

 count given by Cuvier, and also by Dr. Macartney, in Rees' 

 Cyclopedia, article Birds, be correct. 



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 circum- 

 stance 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 sud- 

 den and jerking flexion of the whole limb, or to flexion of 

 the hock-joint alone, I have had no opportunity lately of de- 

 termining. If the latter be the case, it is probably connect- 

 ed 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 na- 

 ture 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 sur- 

 face; 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 pro- 

 jections, which, moving in the fossa, and received into cor- 

 responding depressions in the astragalus, at the moment the 

 limb arrives at the greatest degree either of flexion or of ex- 

 tension. 



The shape of the surfaces of the astragalus concerned in 

 the articulation, is not that a given circle throughout, for to- 

 wards either extremity, the descent is more rapid, or, in 

 other words, answers to an arc of a smaller circle. Hence, 

 when one of the objections 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 con- 

 siderable obstruction to change a 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 pos- 

 terior 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. 



