60 Dr Graves on a PeciiUarity in 



Sheep and cows are not provided with ankle-joints of a simi- 

 lar 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 i« 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, con- 

 cerning the nature and cause of string-halt, a disease to which 

 the sheep and cow are not subject, and we have already observ- 

 ed, that in these animals the structure of this joint presents no- 

 thing remarkable. 



The hock-joint is a good example of what is termed the 

 hinge-like articulation, and is formed between the tibia and as- 

 tragalus, 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 astra- 

 galus, 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 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 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. 



