MUSCLES 215 



delicate adjustment to complicated movements of the hand under 

 the directing power of higher cerebral development. 



Peroneus Tertius. 



The foot of man possesses the small peroneus tertius which was 

 referred to as one of the evertors of the foot concerned in the con- 

 struction of his plantar arch. Macalister and Professor Keith both 

 speak of it as peculiar to man, and the latter refers to it at some 

 length, 1 the whole passage being worth quoting here. " Although 

 the evolution of the human method of progression was attended 

 by a profound alteration in the form and action of every muscle 

 and bone with lower limbs, yet this great transformation was 

 produced without the appearance of any really new element. One 

 new muscle — the peroneus tertius — did appear, and the history of 

 its evolution throws an interesting sidelight on the origin of new 

 structures. It arises by the outer fibres of the common extensor 

 muscles of the toes being separated. In all the anthropoids the 

 feet are so articulated at the ankle-joints that the soles are directed 

 towards each other, and only the outer edge of the foot comes to 

 rest on the ground when the animal tries to stand. The feet have a 

 tendency to assume a similar position in children at birth. The 

 advantage of a muscle, such as the peroneus tertius, is apparent 

 in the human foot, for it tends to raise the outer border of the foot, 

 so that the sole is properly applied to the ground. If we examine 

 the muscles which, rising from the front of the leg, cross the ankle- 

 joint to end on the back of the foot on the toes of fifty men, we shall 

 find every stage in the evolution of this muscle. In one man at 

 least it will be undeveloped ; in two or perhaps three it will be 

 represented by a part of the tendon of the extensor muscle of the 

 little toe, which in place of ending entirely on the toe sends a part 

 to end on the metatarsal bone of the little toe. In only forty of 

 the fifty men will the peroneus tertius be found quite isolated from 

 the parent muscle — the extensor communis digitorum, and to have 

 a. distinct origin from the fibula in the leg, and a separate 

 insertion to the base of the fifth metatarsal bone in the foot. 

 In a series of fifty specimens every stage in the isolation of 

 this new muscle will be seen. It has never been found in 

 any anthropoid, and is more often absent or undeveloped in 

 Afiican than European races." 



To this excellent account I have only to add one comment. 

 It can hardly be an accident or without significance that this special 



1 The Human Body, p. 92. 



