156 COUES ON THE OSTEOLOGY 



Of the Lower Extremities. 



In the upper extremities of the bird, although there is much interesting mechanism 

 displayed, yet the various features are in general only such as are found in the wings of the 

 greater number of the groups of the order. The very peculiar type of structure which 

 stamps such an individuality upon the Colymbidce, Podicipidce, and, to a somewhat less extent, 

 upon the brachypterous Natatores generally, does not extend in any very marked degree 

 into the superior extremities. It is in the morphology of the posterior extremities, both 

 as regards the osseous framework and the muscles by which the bones are moved, that we 

 find the most marked peculiarities of structure. When we recollect how very peculiar are 

 the form and position of the posterior extremities, both as regards the axis of the bird's 

 body, and its centre of gravity, causing progression on land to be awkward and laborious 

 from the stiff and constrained position in which the body must be placed to bring the 

 centre of gravity within the base of support, we might naturally expect to find interesting 

 peculiarities in the structure of these extremities. Such is really the case ; the whole 

 morphology of the posterior limbs being so moulded as to obtain the most advantageous 

 structure possible for propelling the bird over and through the water, at a considerable 

 sacrifice of ease and power of terrestrial locomotion. 



This indication, which we see more or less completely fulfilled throughout the Natatores 

 brachypteri, is perhaps more strongly pronounced among the Coh/mbidce than in any other 

 family of birds, with the exception of course of the Penguins, (Aptenodt/tes, Spheniscus, &c.,) 

 and certainly reaches the maximum of development which is found among any birds that 

 retain the power of aerial locomotion. The species of bird now under consideration being 

 typical, so to speak, of the whole family, in it we can study this peculiar type of structure 

 to great advantage. 



In examining the morphology and functions of the lower extremities, we must of course 

 consider them with reference to their action upon that element to which they are specially 

 adapted, — the water. To produce a perfect pair of paddles, three indications must be 

 fulfilled. First, the limbs must be articulated far backwards, and, while they have the 

 freest motion, must be so abducted from the median line that the broad webs shall not 

 interfere with each other. Secondly, in the backward stroke the broadest possible surface 

 must be attained for action upon the water, while in bringing the leg forward exactly 

 the reverse must be the case. Thirdly, the great power of the legs must be chiefly 

 exerted in pushing ; and hence the bulk of the muscles must be upon the posterior 

 aspect of the several components of the legs. The mode in which this latter indication 

 is fulfilled will be noticed in the description of the muscles. The first two may here be 

 considered. 



Femur. This bone is remarkably short and stout. It measures only two inches in en- 

 tire length, while across the condyles it is three fourths of an inch, or more than a quarter 

 of its total length. The width across the upper extremity, from the trochanter to the 

 o-lobular head, is fully half an inch. The shaft is also exceedingly stout, and much 

 curved, the convexity presenting forward. It is marked along the entire length of its 

 posterior aspect, as well as over the trochanter and on the sides of the condyles, with 

 roughened tubercles and depressions for the attachment of the various muscles which act 

 upon it. 



The globular " caput femoris" is peculiar both in shape and position. It projects almost 



