OSSEOUS SYSTEM. 81 



a very strong ligamentum teres. The Tibia is always a strong and 

 very elongated bone, and is characterized in particular by a bony 

 bridge which proceeds obliquely from its lower end to the external 

 maleolus, and beneath which the tendon of the extensor communis 

 digitorurn passes. Superiorly the tibia is prolonged into a crest- 

 shaped process projecting in the direction upward and forward, 

 above or behind which there lies, as in Aptenodytes, an often large 

 and misshapen patella. In some Palmipedes (Podiceps, Eudytes), 

 this process of the tibia is long, conical, and terminates in a point, 

 and in the latter genus where the patella is wanting, it exceeds 

 even the femur in length. In Podiceps a smaller, similarly formed 

 patella lies behind the process. In the Ostrich two patellae con- 

 stantly occur, situated one above the other. The Fibula is always 

 very slender, applies itself inferiorly to the tibia, to which it is partly 

 united, and is frequently continued below into a mere fibro-carti- 

 lage. The metatarsal or tarso-metatarsal bone very strikingly re- 

 minds us through its simple condition of an analogous structure in 

 the Ruminantia and Solipedia. It is strong and of very great length, 

 projecting inferiorly into three trochlear heads for the three toes, 

 and is frequently present, especially in the Wading-birds, as the 

 principal bone of the leg exceeding remarkably the bones above it in 

 length ; toward its distal extremity a very small adjacent bone is 

 found provided with an articulating head for the great toe when that 

 is present ; it is fixed to the tarso-metatarsal only by a ligament. In 

 the Penguin the latter bone is unusually short and broad, and parti- 

 tioned by deep longitudinal grooves and interspaces, so that its origi- 

 nal division into three metatarsal bones becomes thereby indicated, 

 most birds have four toes, and in that case the large, posterior or 

 inner toe has constantly two, the next to it, three, the middle and 

 longest four, and the external toe five, phalanges. When the 

 great toe is wanting, the remaining digits preserve generally the 

 above number of phalanges, though several departures from this rule 

 occur, as in Caprimulgus, where the external toe has four joints, 

 while in Cypselus all the toes, with the exception of the great toe, 

 have three. 



In Birds there occur more generally than in the other classes of 

 Vertebrate animals, regular ossifications or sesamoid bones in sev- 

 eral of the ligaments and muscular tendons, the presence and func- 

 tional conditions of which frequently characterize whole families 

 and orders. Thus in most birds the lower jaw obtains its supply 

 of air from the tympanic cavity by a membranous tube, which, placed 



6 



