Along the outer side of the foot we can feel--1, the external tuberosity of the os calcis; 2, the external malleolus; 3, the peroneal tubercle of the os calcis, one inch below the malleolus, with the long peroneal tendon below it, and the short one above it; 4, the projection of the base of the fifth metatarsal bone. 125. =Lines of joints.=--In fat persons the following rules for finding the joints may be of service as regards the surgery of the foot:-- The level of the ankle joint lies about half an inch above the end of the inner malleolus. This is worth remembering in performing ‘Syme’s’ amputation. The tubercle of the scaphoid bone is the best guide to the astragalo-scaphoid joint which lies immediately behind it; and the plane of this joint is in the same line as that of the calcaneo-cuboid. Thus a line drawn transversely over the dorsum of the foot, behind the tubercle of the scaphoid, would strike both the joints opened in ‘Chopart’s’ operation. Place your thumb on the tubercle of the scaphoid, and measure about one inch and a half in front: here you find the joint between the internal cuneiform bone and the metatarsal bone of the great toe. This point is useful in Lisfranc’s operation, which consists in the removal of the metatarsal bones. The line of the calcaneo-cuboid joint lies midway between the external malleolus and the (tarsal) end of the metatarsal bone of the little toe. The projection of the fifth metatarsal bone is the guide to the joint between it and the cuboid. Notice that the line of the joints between the metatarsal bones and the first phalanges lies a full inch farther back than the interdigital folds of the skin. This is a point to be remembered in amputating the toes. 126. =Dorsal artery.=--The line of the dorsal artery of the foot is from the middle of the ankle to the interval between the first and second metatarsal bones. The artery can be felt beating over the bones along the outer side of the extensor longus pollicis, which is the best guide to it. 127. =Bursa.=--The synovial sheath of the extensor longus pollicis extends from the front of the ankle, over the instep (apex of the internal cuneiform bone) as far as the metatarsal bone of the great toe. There is generally a bursa over the instep, above, or it may be, below, the tendon. There is often a large irregular bursa between the tendons of the extensor longus digitorum, and the projecting end of the astragalus, over which the tendons play. There is much friction here. It is well to be aware that this bursa sometimes communicates with the joint of the head of the astragalus.