J^ 



88 



wise 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 con- 

 tour 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 stretch- 

 ed, 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 circumfe- 

 rence ; but as soon as the tibia is moved beyond this point of inac- 

 tion 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 articulating 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 particu- 

 larly 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. 



In the autumn of 1829, two of the species of whale called Del- 

 phinus diodon, by Hunter, Hyperoodon, by La-Cepede, and Ceto- 



