OSSEOUS SYSTEM OF AVES. 79 



thus the distal end of the tibia is like that of the femur mth 

 the back of the condyles turned forward, and without the notch 

 in either. 



Among the modifications of the proximal end of the tibia may 

 be noted the production of the rotular process in the axis of the 

 shaft two inches above the knee-joint in the Divers ( Colymbus), 

 fig. 34, /i; both pro- and ecto-cnemial ridges descend from the 

 fore part of the base of this process, the former extending half- 

 way down the shaft of the tibia. In the Albatross the pro- and 

 ecto-cnemial ridges are much developed ; but are still more so in 

 the extinct Cnemiornis, without corresponding production of the 

 rotular or ' epicnemial ' process. In the Ostrich this process 

 extends forward, without rising above the level of the proximal 

 surface, and contracting to its termination there divides into small 

 pro- and ecto-cnemial processes; the latter the shortest and 

 tuberous. The distal condyles are less produced anteriorly, com- 

 mence more abruptly and are more produced posteriorly, than in 

 other birds : their articular surfaces are so continuous as to leave 

 no ' intercondylar ' space ; there is no tendinal groove or bridge : 

 but a tuberosity above the middle of the confluent condyles. 

 The articular surface of these being concave in one direction, 

 convex in the other, forms a ' trochlea,' and the same in the con- 

 joined parts of the distal condyles in other birds. It limits the 

 movements of the next segment of the limb to one plane. 



The Jihula, fig. 34, 67, is a styliform bone ending in a point below 

 at various distances down the tibia in different birds. The articular 

 head is subcompressed, convex in the longer axis, slightly curved 

 backward, hollowed on the inner (tibial) side : rather convex 

 externally : the shaft shows the rough linear tract for attachment 

 to the tibia: and there are sometimes tuberosities for tendinal 

 insertions on the opposite side. 



The femur is ossified from one centre: the tibia has an epiphysis 

 for the distal condyles ; the proximal end of the metatarsus is 

 ossified from one centre, forming an epiphysis which caps the ends 

 of the three metatarsals that coalesce, first with each other, then 

 A^dth the epiphysis, to form the single compound bone. 



The trochlear epiphysis of the tibia most resembles the astra- 

 galus in those mammals (Ruminants, e.g.) in which the meta- 

 tarsals coalesce. The term ' tarso-metatarse ' applied by some 

 ornithotomists to the present segment, fig. 34, 68, implies the tarsal 

 homology of the epiphysis ; the same might, more probably, be 

 predicable of the distal one of the tibia ; but neither being demon- 

 strated, I prefer to call the present segment the ' metatarse.' It 



