FOURTH LAYER OF MUSCLES. 625 



surfaces of those bones ; and are inserted partly into the tibial side of the 

 base of the metatarsal phalanx of the same toes, and partly by an expan- 

 sion from each to the extensor tendons on the dorsum of the phalanx. 

 These muscles are smaller than the dorsal, and are placed more in the 

 sole of the foot. 



The dorsal interossei, i, one in each space, arise by two heads from the 

 lateral surfaces of the bones between which they lie ; and are inserted like 

 the others into the side, and on the dorsum of the metatarsal phalanx of 

 certain toes : Thus, the inner two muscles belong to the second toe, one to 

 each"side ; the next appertains to the outer side of the third toe ; and the 

 remaining one to the outer side of the fourth toe. 



The interossei are crossed by the external plantar artery and nerve, and 

 their digital branches, and lie beneath the transversalis pedis and the meta- 

 tarsal ligament. The posterior perforating arteries pierce the hinder ex- 

 tremities of the dorsal set. 



Action. Like the interossei of the hand (p. 281) they will contribute to 

 the bending of the metatarso-phalangeal joints by the flexors, and will help 

 the extensors to straighten the last two phalangeal joints. 



They can act also as abductors and adductors of the toes. Thus the 

 plantar set will bring the three outer towards the second toe ; and the 

 dorsal muscles will abduct from the middle line of the second toe the two 

 attached to that digit moving it to the right and left of the said line. 



Dissection. Following the tendon of the tibialis posticus muscle from 

 its position behind the inner malleolus to its insertion into the scaphoid 

 bone, trace the numerous processes that it sends forwards and outwards. 

 Open also the fibrous sheath of the tendon of the peroneus longus, which 

 crosses from the outer to the inner side of the foot. 



The tendon of the TIBIALIS POSTICUS is continued forwards over the 

 internal lateral ligament of the ankle joint, and over the astragalo scaphoid 

 articulation to be inserted into the prominence of the scaphoid bone. 

 From its insertion processes are continued to many of the other bones of 

 the toot : One is directed backwards to the margin of the groove in the os 

 calcis for the tendon of the flexor longus pollicis. Two offsets are directed 

 forwards ; one to the internal cuneiform bone; the other, much the largest, 

 is attached to the middle and outer cuneiform, to the os cuboides, and to 

 the bases of the second, third, and fourth metatarsal bones. In other words, 

 pieces are fixed into all the tarsal bones except one (astragalus) ; and into 

 all the metatarsal bones except two (first and fifth). 



Where the tendon is placed beneath the articulation of the astragalus, 

 it contains a sesamoid bone, or fibro-cartilage. 



The tendon of the PERONEUS LONGUS MUSCLE winds round the cuboid 

 bone, and is continued inwards in the groove on the under surface, to be 

 inserted into the internal cuneiform bone, and the base of the metatarsal 

 bone of the great toe ; and sometimes by a slip into the base of the second 

 metatarsal bone. 



In the sole of the foot (fig. 213), it is contained in a sheath which is 

 crossed, towards the outer part, by the fibres of the long plantar ligament 

 prolonged to the tarsal ends of the third and fourth metatarsal bones ; but 

 it is formed internally only by areolar tissue. A separate synovial mem- 

 brane lubricates the sheath. 



Where the tendon turns round the cuboid bone it is thickened, and con- 

 tains fibro-cartilage or a sesamoid bone. 

 40 



