490 MYOLOGY 



only to the level of the base of the fifth metatarsal. On the medial side of the ankle 

 (Fig. 442) the sheath for the Tibialis posterior extends highest up to about 

 4 cm. above the tip of the malleolus while below it stops just short of the tuber- 

 osity of the navicular. The sheath for Flexor hallucis longus reaches up to the level 

 of the tip of the malleolus, while that for the Flexor digitorum longus is slightly 

 higher; the former is continued to the base of the first metatarsal, but the latter 

 stops opposite the first cuneiform bone. 



On the lateral side of the ankle (Fig. 441) a sheath which is single for the greater 

 part of its extent encloses the Peronsei longus and brevis. It extends upward 

 for about 4 cm. above the tip of the malleolus and downward and forward for 

 about the same distance. 





IV. THE MUSCLES AND FASCLffi OF THE FOOT. 



1. The Dorsal Muscle of the Foot. 



Extensor digitorum brevis. 



The fascia on the dorsum of the foot is a thin membranous layer, continuous 

 above with the transverse and cruciate crural ligaments; on either side it blends 

 with the plantar aponeurosis; anteriorly it forms a sheath for the tendons on the 

 dorsum of the foot. 



The Extensor digitorum brevis (Fig. 441) is a broad, thin muscle, which arises 

 from the forepart of the upper and lateral surfaces of the calcaneus, in front of 

 the groove for the Peronseus brevis; from the lateral talocalcanean ligament; 

 and from the common limb of the cruciate crural ligament. It passes obliquely 

 across the dorsum of the foot, and ends in four tendons. The most medial, which 

 is the largest, is inserted into the dorsal surface of the base of the first phalanx of 

 the great toe, crossing the dorsalis pedis artery; it is frequently described as a 

 separate muscle the Extensor hallucis brevis. The other three are inserted into 

 the lateral sides of the tendons of the Extensor digitorum longus of the second, 

 third, and fourth toes. 



Variations. Accessory slips of origin from the talus and navicular, or from the external cunei- 

 form and third metatarsal bones to the second slip of the muscle, and one from the cuboid to the 

 third slip have been observed. The tendons vary in number and position; they may be reduced 

 to two, or one of them may be doubled, or an additional slip may pass to the little toe. A super- 

 numerary slip ending on one of the metatarsophalangeal articulations, or joining a dorsal inter- 

 osseous muscle is not uncommon. Deep slips between this muscle and the Dorsal interossei occur. 



Nerves. It is supplied by the deep peroneal nerve. 



Actions. The Extensor digitorum brevis extends the phalanges of the four toes into which 

 it is inserted, but in the great toe acts only on the first phalanx. The obliquity of its direction 

 counteracts the oblique movement given to the toes by the long Extensor, so that when both 

 muscles act, the toes are evenly extended. 



2. The Plantar Muscles of the Foot. 



Plantar Aponeurosis (aponeurosis plantaris; plantar fascia). The plantar apo- 

 neurosis is of great strength, and consists of pearly white glistening fibers, disposed, 

 for the most part, longitudinally: it is divided into central, lateral, and medial 

 portions. 



The central portion, the thickest, is narrow behind and attached to the medial 

 process of the tuberosity of the calcaneus, posterior to the origin of the Flexor 

 digitorum brevis; and becoming broader and thinner in front, divides near the 

 heads of the metatarsal bones into five processes, one for each of the toes. Each 

 of these processes divides opposite the metatarsophalangeal articulation into two 

 strata, superficial and deep. The superficial stratum is inserted into the skin of 



