86 PHYSIOLOGICAL SERIES. 



B. 187. Left ankle-joint of a Three-toed Sloth (Bradypus tridac- 

 tylus}. The cavity of the joint has been opened in front to 

 show a curious peg and socket arrangement by which the 

 inturning of the sole by the powerful tibialis anticus is 

 facilitated, and outward displacement of the foot during this 

 movement prevented. The peg is formed by a conical 

 extension of the lower extremity of the fibula ; its point is 

 directed diagonally inwards and downwards, and fits into a 

 corresponding concavity in the upper and outer surface of 

 the astragalus. A somewhat similar contrivance, though 

 far less strongly marked, is found in certain Monkeys, 

 e. g. Nasalis, in which the sole of the foot is inturned in 

 climbing. 



Humphry, Jour, of Anat. & Physio!., vol. iv. 1870, p. 22. 



B. 188. Right ankle-joint of a Spider Monkey {Ateles melanocliir). 

 The external lateral ligament is in three fasciculi similar to 

 those present in Man. The posterior portion of the capsular 

 ligament has been reflected upwards to show the inter- 

 articular cartilage attached to it. 



B, 189. Lower ends of left tibia and fibula of a Spider Monkey 

 (Ateles melanochir) , showing the interarticular cartilage 

 projecting from the posterior part of the capsule. 



B. 190. Right ankle-joint of Paplo [ Cynoccphalus] sphinx. The 



external lateral ligament is in two fasciculi, corresponding 



respectively to the middle and posterior fasciculus of Man. 



Presented by Prof. F. G. Parsons. 



