OSSEOUS SYSTEM OF AVES. 83 



one is sliort and directed backward. The hind toe articulates 

 on the same plane as the otliers in grasping and perching birds, 

 but on a higher level in terrestrial and aquatic kinds. By the 

 analogy of the number of the phalanges of these toes with those 

 in Lizards (vol. i. p. 192, fig. 122) the back toe, fig. 34, z, is the 

 innermost, answering to ^ hallux ; ' the inner of the front toes, ib. 

 ii, is the second ; the middle one, ib. Hi, is the third : the outer 

 one, ib. iv, is the fourth : it will be seen that the number of 

 phalanges progressively increases from two to five. The fifth 

 toe of the four phalanges in the Monitor is not developed in any 

 bird. The constancy of the numl^er of phalanges in each toe is 

 such that the toes retained in a tridactyle bird, e. g.. Emeu, are 

 seen to be the second, third, and fourth; those in a didactyle 

 bird, e. g., Ostrich, to be the third and fourth : and, although the 

 latter is much the smaller and shorter toe, it retains its superior 

 number of joints. Among the very few" exceptions to this rule 

 may be cited the outer toe of the Caprimulgus and of the Swift, 

 which has but four phalanges ; in the Swift, also, the innermost 

 toe is directed forward like the rest. The last phalanx in each 

 toe is pointed, and usually curved, corresponding in some measure 

 with the shape of the claw it supports : the articular ends of the 

 phalanges are slightly expanded and coadapted with trochlear 

 joints limiting motion to one plane. 



The chief of the sesamoid bones in the hind limb is the patella : 

 it is of unusual size in the Penguin, is ossified from two centres, 

 and articulates with the procnemial process of the tibia: it 

 coexists with the long rotular process in the Loon, fig. 34, / ; it 

 is large and of an angular form in the Musk-duck {Biziurd) : in 

 the Merganser the patella is largest and deeply notched ; in the 

 Coot it is elongate. In most aerial birds a patella is wanting. 

 A calcaneal sesamoid is wedged into the outer and back part of 

 the ankle-joint in the Apteryx, and plays upon the back part of 

 the tibial trochlea in the Turkey, Guan, Curassow, and some 

 other Rasores. 



Ossification normally extends into the tendons of some of the 

 muscles in most birds : e. g., of the deep seated spinal ones of the 

 back ( Uria Troile and many others) ; of the muscles of the foot 

 and toes (^GaUince). The bony plates at the corneal margin of 

 the sclerotic tunic of the eye, and the columelliform stapes of the 

 ear, are appendages to sense-organs. Mr. William Home Clift 

 discovered small ossifications at the attachments of the semilunar 

 valves of the aorta and pidmonary artery in some Birds. ^ 



' VTT-, p. 331. 

 g'2 



