The Tarsus. The tarsus of the dog consists of seven bones arranged in two rows, of two and four respectively, with a centrale between them. The two bones of the proximal row are the astragalus and calcaneum.
The astragalus (fig. 80, 10) corresponds to the fused tibiale and intermedium of the typical tarsus. Its proximal end is much wider than its distal end, and forms a large rounded condyle articulating with the tibia, while its posterior end meets the navicular. It lies to the dorsal side of the foot.
The calcaneum (fibulare) (fig. 80, 11), the thickest bone in the pes, lies somewhat behind, and to the outer side of the astragalus. It articulates with the astragalus and fibula, and is drawn out behind into a long rounded process, which forms the heel, and is in the young animal terminated by an epiphysis. Between the proximal and distal rows of tarsals is the navicular (centrale) (fig. 80, 12), a somewhat flattened and square bone articulating with the astragalus.
The distal row of tarsals consists of four bones. The internal cuneiform (tarsale 1) is a smooth flattened bone lying to the inner side of the foot; it articulates with the first metatarsal and with the navicular. The middle cuneiform (tarsale 2) (fig. 80, 13) is a still smaller bone, lying external to the internal cuneiform. It articulates with the second metatarsal and with the navicular. The external cuneiform (tarsale 3) (fig. 80, 14) is a larger, somewhat square bone lying external to the middle cuneiform. It articulates with the third metatarsal and with the navicular. The cuboid (tarsalia 4 and 5) (fig. 80, 15) is a considerably larger bone lying to the outer side of the foot. It articulates with the fourth and fifth metatarsals and with the calcaneum.
The pes has sometimes five digits, sometimes four, the hallux being absent. Even when present the hallux (fig. 80, B, I) is commonly much reduced, and may be quite vestigial, and represented only by a small nodular metatarsal.
Each of the other digits consists of a long metatarsal, which in the young animal has a prominent epiphysis at its distal end, and of three phalanges. The proximal and middle phalanges have epiphyses at their proximal ends, while the distal phalanx is without epiphyses and is claw-shaped.
CHAPTER XXII.
GENERAL ACCOUNT OF THE SKELETON IN MAMMALIA.
THE EXOSKELETON AND VERTEBRAL COLUMN.