THE LOWER LIMB 553 



therefore, cross each other, that of the flexor longus hallucis being 

 above, or on the deep surface of, the other, and giving a slip to it. 

 The tendon of the flexor longus digitorum, on reaching the middle 

 line of the sole, receives the flexor or musculus accessorius, and 

 thereafter terminates in four tendons for the four outer toes, with 

 which tendons the lumbricales are associated. 



Flexor or Musculus Accessorius. — This muscle is so named 

 because it is accessory to the long flexor. It arises by two heads, 

 inner and outer, which embrace between them the os calcis and the 

 long plantar ligament. 



Origin. — ^The inner head arises from the internal concave surface 

 of the OS calcis below the groove on the under surface of the susten- 

 taculum tali. The outer head arises from (i) the junction of the 

 external and plantar surfaces of the os calcis in front of the outer 

 tubercle ; and (2) the adjacent portion of the long plantar ligament. 



Insertion. — ^The outer border and upper surface of the tendon 

 of the flexor longus digitorum about the centre of the sole. 



Nerve-supply. — The external plantar nerve. 



Action. — (i) To flex the terminal phalanges of the four outer toes, 

 and (2) to counteract the tendency of the long flexor tendons to 

 draw the toes inwards during flexion. In performing this latter 

 action, the muscle keeps the long flexor tendon in the middle line, 

 so that its four divisions act in a straight line upon the toes. 



The inner head of the muscle is broad and fleshy, whilst the outer 

 head is narrow, pointed, and tendinous. 



This muscle is to be regarded as a detached portion of the 

 flexor longus digitorum, forming an additional tarsal origin for the 

 purpose of counteracting the obliquity of its tendons. 



Lumbricales. — These muscles are four in number. 



Origin. — From the tendons of the flexor longus digitorum where 

 these begin to diverge, the outer three taking origin each from the 

 contiguous sides of the tendons between which it lies, and the most 

 internal springing only from the inner side of the long flexor tendon 

 destined for the second toe. 



Insertion. — ^The tendons, having passed round the inner sides of 

 the metatarso-phalangeal joints of the four outer toes, take insertion 

 each into the expansion formed by the extensor tendon on the 

 dorsal aspect of the first phalanx. 



Xerve-siippiy. — ^The most internal or first lumbricalis is supplied 

 by the internal plantar nerve, more particularly by its second 

 digital branch on its way to the cleft between the great toe and the 

 second. The outer three lumbricales, or the second, third, and 

 fourth, are supplied by the deep division of the external plantar 

 nerve. 



Action. — (i) To flex the metatarso-phalangeal joint, and (2) to 

 extend the interphalangeal joints. 



The muscles are to be regarded as detached portions of the flexor 

 longus digitorum. 



Third Layer. — ^The third layer consists of the following four 



