Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/741

ending in a mere style below. Generally, it is decidedly shorter than the tibia, but it has the same length as that bone in some Penguins. The tibia (t.), or rather "tibio-tarsus," is a highly characteristic bone. Its proximal end is expanded, and produced anteriorly, into a great cnemial process (which may be variously subdivided), as in Dinosauria. The distal end is pjtx terminated by a well-marked pulley-like articular surface, which is inclined somewhat forwards as well as downwards. Not unfrequently there is an oblique bar of bone on the anterior face, just above the pulley, beneath which the long ex

tensor tendons pass.

The extremity of the cnemial process in Struthio, Rhea, and Dromceus is ossified as an epiphysis ; and in young birds the whole of the distal articular end of the bone is separated from the rest by a suture, and also appears tc be an epiphysis. But it is, in fact, as Professor Gegenbaur^[1] has proved, the proximal division of the tarsus (apparently representing only the astragalus of the other Vertebrata], which exists in the embryo as a separate car tilage, and, as it ossifies, ankyloses with the tibia. The so-called tibia of a bird is therefore, properly speaking, a tibio- tarsus.

In all Birds, even in Archoeopteryx, the fifth digit of the pes remains undeveloped;^[2] and the second, third, and fourth metatarsals are ankylosed together, and by their proximal ends, with a bone, which is a distinct cartilage in the foetus, and represents the distal division of the tarsus. Thus a tarso-metatarsus is formed (fig. 37). The distal ends of the metatarsals remain separate, and offer convex articular surfaces to the proximal phalanges of the digits.

In the Penguins, large apertures lie between the several metatarsals of the adult tarso-metatarsus; and in other birds more or less considerable passages persist between the middle and lateral metatarsals proximally, and the middle and outer distally. In most birds the middle metatarsal does not remain parallel with the others, but its proximal end inclines a little backward, and its distal end a little forward. Hence the two apertures on each side of its proximal end may lie at the bottom of a fossa, or run into one in front, while they remain distinct behind.

Again, in most Birds the posterior face of the proximal end of the middle metatarsal, and the adjacent surface of the tarsal bone, grow out into a process which is commonly, but improperly, termed " calcaneal." The inferior surface of this hypo-tarsus is sometimes simply flattened, some times traversed by grooves or canals for the flexor tendons of the digits.

When a hallux exists, its metatarsal bone is usually in complete above, and is united to the ligament by the inner or the posterior surface of the tarso-metatarsus. In the Frigate-bird (Phaethon), and in Steatornis, the hallucal meta tarsal is remarkably long. The genus Phaethon stands alone, as far as we know, in having the hallucal metatarsal ankylosed with the others.

In many of the Alectoromorphce a spur (calcar), consisting of a bony core ensheathed in horn, is developed on the inner side of the metatarsus, and becomes ankylosed with the metatarsal of the second digit ; in some there are two spurs. In a few birds, similar spurs (Palamedea), or osseous excrescences (Pezophaps), are developed in relation with the metacarpus.

The normal number of the pedal phalanges in Birds is (as in ordinary LacertUia) two, three, four, five, reckoning from the hallux to the fourth digit. Among the few Birds which constitute exceptions to the rule are the Swifts, in which the third and fourth toe have only three phalanges each (2, 3, 3, 3), and the Goatsuckers, in which the fourth toe only has the number reduced (2, 3, 4, 4) not 2, 3, 4, 3, as Professor Huxley (op. Y.)puts it for Caprimulgm. Mr Parker has figured the fourth toe of the Sand-grouse (Syrrhaptes) with only three, but he speaks (op. dt., p. 203) of only one as wanting in that toe.

Many Birds have only three toes by suppression of the

hallux. In the Ostrich, not only the hallux, but the phalanges of the second digit, are suppressed, and the

distal end of the second metatarsal is reduced to a mere

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1. See " Archiv fiir Anat.," in Phys. Jalirgang, 1863, and Huxley, on " Dinosaurs," Q. Jour. Geol. Soc., Nov. 10, 1869.
2. In his earlier papers, Mr Parker mistook the bony core of the cock s spur for the first, thus making the proper hallux the second toe.