OF COLYMBUS TOKQUATUS. 159 



entire surface of the spine presents roughened elevations and depressions for the attach- 

 ment of muscles. The oval concave head of the fibula, placed on the external aspect, 

 completes the crural elements of the knee-joint. 



Just at the superior border of the joint there is a very small projecting process of bone, 

 which is generally regarded as the true analogue of the patella. 



The twofold design of this elongated spine is quite evident. In the first place, by the 

 simplest mechanical law, it increases greatly the power of the extensor muscle the 

 cruraeus, coming down over the front of the femur. Secondly, some of the most power- 

 ful of the muscles which act upon the metatarsus take their origin primarily from its 

 extreme tip, and the consequent increased length of fleshy bellies which they thus 

 acquire gives them an equal increase of space for contraction. 



The construction and movement of the knee-joint jaresent several interesting peculiari- 

 ties. Although both condyles of the femur articulate with the tibia, the most extensive 

 articulation of the outer one is with the head of the fibula, upon which it principally 

 rests. In this bird, however, while flexion and extension of course form the most promi- 

 nent elements in the motion of the leg upon the thigh, to these there is superadded a very 

 considerable degree of true rotatory motion, also quite different from the peculiar obliquity 

 of motion which obtains in the elbow-joint. 



The ordinary position of the leg, with reference to the axis of the body, when least 

 influenced by muscular action, is to lie not very far from horizontally backwards, with 

 some degree of inclination downwards, and a greater amount of obliquity outwards. 

 When the leg is strongly flexed, it results, from the peculiar direction of the femur 

 from the body and from the plane of the femoral condyles, that the leg becomes quite par- 

 allel with the axis of the body, while its outward inclination is totally lost. The leo-s in a 

 fresh specimen may be easily made to touch each other over the coccygeal vertebra?. Con- 

 versely, with strong extension, the axis of the leg becomes more and more nearly perpen- 

 dicular to the axis of the spine, while its amount of abduction from the body is constantly 

 augmented. These are the positions consequent upon simple flexion and extension. 



In the ordinary semi-flexed position of the leg, the plane of the condyles with which 

 the metatarsus articulates, is placed obliquely outwards instead of directly forwards, so 

 that the metatarsus projects outwards from the body; and the fibula, especially at its 

 upper part, is rather upon the posterior than the directly external aspect of the leg. 

 Now, when the leg is drawn forwards, the same muscles are so inserted that they cause a 

 rotatory motion (chiefly in the knee-joint), which brings the fibula directly outwards, the 

 plane of the tibial condyles directly forwards, — and consequently the sharp anterior edge 

 of the metatarsus, and the anterior surface of the closed toes, cut easily through the water, 

 with but slight resistance. 



Although the femur participates in this rotatory motion of the lower extremities, yet the 

 rotation is still greater in the knee-joint. From the construction of the joint, — remem- 

 bering the broad flat head of the tibia on which the convex internal condyle rests, and the 

 much more intimate coaptation of the head of the fibula with the external condyle, — it 

 seems probable that the extremity of the fibula is chiefly the pivot around which the 

 motion of the knee-joint takes place. Several muscles of the leg seem to act solely as 

 rotators, while many of the flexors and extensors are so inserted as to have some such 

 action. 



The two condyles at the distal extremity of the tibia forming the "malleoli," are very 



MEMOIRS BOST. SOC. NAT. HIST. Vol. I. Ft. 2. 41 



