THE INTEROSSEOUS MUSCLES. 273 



The dorsal interosseous muscles lie one in each interspace. Each muscle arises 

 by two heads from the adjacent sides of two metatarsal bones ; their muscular fibres 

 are attached pinnately on the two sides of a central tendon, which is inserted partly 

 into the base of the first phalanx and partly into the extensor aponeurosis of the toe 

 to which the muscle belongs. The first two are inserted one on each side of the 

 second toe ; the third and fourth are severally inserted on the outer sides of the 

 third and fourth toes. At the proximal end of each muscle there is an interval 

 between the two origins giving passage to perforating arteries as in the hand. 



The inner head of the first dorsal interosseous muscle is smaller than the others, 

 and arises from the base of the first metatarsal bone and the dorsal edge of the 

 internal cuneiform. The third and fourth muscles also receive fibres posteriorly 

 from the sheath of the peroneus longus. 



The plantar interosseous muscles arise from the inner and under surfaces of the 

 third, fourth and fifth metatarsal bones, and from the sheath of the peroneus longus ; 

 they are severally inserted, in a manner similar to the dorsal interossei, partly into 

 the inner sides of the first phalanges of the third, fourth and fifth toes, and partly 

 into the extensor tendons of these toes. 



Bursse are generally present between the heads of the metatarsal bones, often between the 

 tendons of the plantar interosseous muscles and the metatarso-phalangeal articulations, and 

 occasionally beneath the tendons of the second and third dorsal interosseous muscles 

 (Gruber). 



In the f O3tus the dorsal interosseous muscles are originally situated on the plantar aspect of 

 the closely appressed metatarsal bones and have only a single origin, which corresponds to the 

 outer head of the fully formed muscle. The metatarsal bones then gradually become 

 separated, and the muscles make their way in the intervals to the dorsum of the foot, acquiring 

 at the same time their attachment to the inner bone, thus becoming two-headed. The 

 definitive condition is attained by the twelfth week. In many animals these muscles are 

 situated permanently in the sole. Similar changes take place in the muscles of the hand. 

 (Huge, Morph. Jahrb., iv, 1878.) 



Nerves. The extensor brevis digitorum is supplied by the anterior tibial nerve. Of the 

 muscles of the sole, the flexor brevis digitorum, the abductor and flexor brevis hallucis, and 

 the innermost lumbricalis are supplied by the internal plantar nerve ; all the others, 

 including the flexor accessorius and the outer three lumbricales, are supplied by the external 

 plantar nerve, 



Actions of the muscles of the leg and foot. There is only one muscle of the leg, viz., 

 the popliteus, which acts on the knee-joint alone. Its principal action is that of a rotator 

 inwards of the leg, but it also assists in bending the knee. It comes into play especially in 

 the commencement of flexion, producing the inward rotation of the tibia (or outward rotation 

 of the femur) which is essential to that stage of the movement. 



It may here be observed that owing to the difference in the direction of the bend at the 

 ankle, as compared with that at the wrist- joint, some confusion is apt to arise in the descrip- 

 tion of the action of the muscles of the leg upon the foot. As it is obvious from morpho- 

 logical considerations that the anterior surface of the leg and the dorsal surface of the foot 

 belong to the general extensor surface of the limb, corresponding with the back of the fore- 

 arm and the dorsum of the hand ; and that the back of the leg and the sole of the foot 

 belong in the same way to the general flexor surface corresponding with the front of the 

 forearm and the palm of the hand, the foot being in a position of over-extension, it follows 

 that the muscles on the anterior aspect of the leg are really extensors, and that their action 

 in moving the foot so as to approximate the toes to the front of the leg merely increases the 

 over-extension ; while, on the other hand, the muscles on the posterior aspect of the leg are 

 really flexors, and their action in straightening the ankle-joint is merely to reduce this over- 

 extension by an opposite movement of flexion. Nevertheless, it has been customary to apply 

 the name of flexion to the action of raising the foot on the front of the leg, and that of exten- 

 sion to its depression. In the following description the usual terms are employed. 



The gastrocnemius acts both as a flexor of the knee and an extensor of the ankle-joint. 

 When the anterior muscles of the leg fix the ankle-joint, it is fitted to act as a flexor of the 

 knee ; when the knee is fixed either by being placed in complete extension or by the 

 sustained action of the extensor muscles, the gastrocnemius acts entirely on the foot, 

 and combines with the solcus in lifting the heel from the ground, and in raising the body on 

 the toes. 



The tiUalls anticus and peroneus tertius are flexors of the ankle ; the tibialis posticus and 



