1867.] 271 [Wyman. 



the Grebes (Fig. 13, C), Loons, Penguins, Gannets, etc., can be 

 brought forward with far more force to show that the tibia is the hom- 

 otype of the ulna, for, in addition to its resemblance to an olecranon 

 in form, " it affords extensive attachments hy way of insertion to the 

 extensors of the tibia," * which the fibula does not. 



If the fibula is the homotype of the ulna, then it follows that the 

 flexor muscles of the leg, which are inserted into the upper end of it, 

 are the homotypes of the extensor muscles of the fore arm, which are 

 inserted into the olecranon ; with this comes another difficulty grow- 

 ing out of the fact that the internal angle formed by the leg and thigh, 

 and which is on the side of flexion, corresponds with, or is the homo- 

 type of, the external angle formed by the arms and fore arm, which is 

 on the side of extension. 



Prof Owen derives additional support to his view as to the homol- 

 ogy of the ulna and fibula, from the mode of articulation in some 

 animals of the tibia with the fibula, as compared with that of the ulna 

 and radius. " The correspondence of the fibula with the ulna is very 

 remarkably maintained in the Petaurus taguanokles, in which the 

 proximal articular surfiice of the fibula is divided into two facets, one 

 playing upon the outer condyle of the femur, the other concave verti- 

 cal and receiving an adapted convexity on the outer end of the head 

 of the tibia, which rotates thereupon exactly like the radius in the 

 lesser sigmoid cavity of the ulna."f 



In connection with this statement it may be remarked that as re- 

 gai'ds the articulation of the fibula with the femur, it is the exception 

 among mammals to find it coming in contact with that bone, but even 

 in Monotremes, Birds and Reptiles, the extent of its articulation with 

 the femur is always secondary to that of the tibia, and never becomes 

 as extensively articulated Avith it as the ulna does with the humerus. 

 As regards the rotation of one bone on the other, we have found, 

 after a careful examination of the parts in an Ornitliorhynchiis which, 

 through the kindness of Prof Agassiz, we have recently had an op- 

 portunity to dissect, that it is the fibula which i-otates, while the 

 tibia is fixed, and in this respect the latter more nearly resembles 

 the ulna. The fibula in the Petaurista resembles the ulna in having a 

 si<Tmoid notch. This cannot be considered as a decisive character, 

 when it is remembered that the articulating surface of the lower jaw 

 is convex in mammals, but concave in other vertebrates ; the lower 

 jaw does not in consequence change its homology in these last. 



Admitting all the arguments which have been adduced to prove 

 the fibula to be the homotype of the ulna, we have still another and 

 greater difficulty than any thus far mentioned, growing out of its rel- 



* Owen Cyclop. Anat. and Physiol. Vol. i, p. 287. 



t Cyclop. Anat. and Physical. Art. llarsupialia. p. 285. 



