The =fibula= (fig. 79, C) is a distinct but very slender bone, somewhat expanded at both ends. It lies external to the tibia and articulates by its proximal end with the head of the tibia, and by its distal end with the calcaneum. Its shaft and proximal end ossify from one centre, and its distal end from a second. The =Pes=. The structure of the =pes= corresponds closely with that of the manus. It is divided into:-- _a._ The =tarsus= or =ankle= formed of a group of small bones. _b._ The =foot=, which includes, firstly, some elongated bones, the =metatarsals=, forming what corresponds to the sole of the foot, and secondly the =phalanges=, which form the toes. 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. FOOTNOTES: