MORPHOLOGY OF LIMBS. 245 
SESAMOID BONES. 
As in the hand, small independent nodules of bone, called sesamoid bones, are met 
with in the ligaments and tendons of the foot. The most constant of these are found in 
connexion with the metatarso-phalangeal articulation of the great toe, where they lie in 
grooves on the under surface of the head of the metatarsal bone in connexion with the 
tendons of the short muscles of the great toe. Small osseous nodules occupying a corre- 
sponding position are occasionally met with in the other toes, and instances have been 
recorded of like ossicles occurring on the plantar aspect of the interphalangeal joint of 
the great toe. 
An osseous nodule is not infrequently met with in the tendon of the peroneus longus 
as it turns round the outer border of the foot to lie in the groove on the under surface of 
the cuboid. 
MORPHOLOGY OF LIMBS. 
MorPHOLOGY OF THE APPENDICULAR SKELETON. 
The paired limbs first appear in the human embryo about the third week as small buds on 
either side of the cephalic and caudal ends of the trunk. That these outgrowths are derived from 
a large number of trunk segments is assumed on the ground that they are supplied by a corre- 
sponding number of segmental nerves, and the circumstance that they are more particularly 
associated with the ventral offsets of these nerves would point to the conclusion that they belong 
rather to the ventral than the dorsal surface of the body. 
At first the surfaces of these limb buds are so disposed as to be directed ventrally and dorsally, 
the ventral aspect corresponding to the future flexor surface of the limb, the dorsal to the ex- 
tensor side. At the same time, the borders are directed headwards (pre-axial), and tailwards 
(post-axial). As the limbs grow, they soon display evidence of division into segments correspond- 
ing to the hand and foot, forearm and leg, upper arm and thigh. Coincident with this (about the 
second month) the cartilaginous framework of the limb is being differentiated. The disposition 
of these cartilages furnishes a clue to their homologies. In the fore limb, the radius and thumb 
lie along the pre-axial borders, and correspond to the tibia and great toe, which are similarly dis- 
posed in the hind limb ; whilst the ulna and fifth finger are homologous with the fibula and fifth 
toe, which are in like manner arranged in relation to the posterior (post-axial) border of their 
respective limbs. Up to this time the limbs are directed obliquely ventralwards from the head 
towards the tail-end of the embryo. During the third month, however, a change in their posi- 
tion takes place, owing to their axes being rotated in opposite directions. The fore limb is 
turned outwards and forwards to the extent of 90°, whilst the lower hind limb is twisted inwards 
and backwards to a corresponding degree. This gives rise to a change in the disposition of the 
joints of the flexor and extensor surfaces. The flexor surface of the elbow is now directed for- 
wards, whilst the corresponding aspect of the knee is turned backwards, and in consequence the 
dorsal or extensor aspect of the fore limb is posterior, whilst the dorsal or extensor aspect of the 
hind limb has become anterior. Correspondingly, the pre-axial border of the fore limb with the 
thumb now lies external, whilst the pre-axial border of the hind limb with the great toe has 
become internal. A knowledge of these changes is necessary to account for the homologies of the 
various structures within the limb. In the axial mesoderm of each member, differentiation 
into cartilaginous segments begins about the second month; each of these cartilages becomes in- 
vested by a perichondrial layer which stretches from segment to segment, and ultimately forms 
the ligaments surrounding the joints, which are subsequently developed between the segments. 
Chondrification first begins in the basal part of the limb, and extends towards the digits. In the 
upper arm and thigh the humerus and femur are homodynamous. In the forearm and leg the 
_ pre-axial radius corresponds with the pre-axial tibia, and the post-axial ulna with the post-axial 
fibula. The homodynamy of the carpal and tarsal elements may be tabularly expressed, and 
~ compared with the more generalised types from which they are evolved. 
Type. Hand. Foot. 
Radiale (Tibiale) =Scaphoid (body) = Astragalus. 
Intermediim =Semilunar = Absent, or Os trigonum (?) 
Ulnare (Fibulare) = Cuneiform = Os Calcis. 
Centrale = Absent, or Tubercle on Scaphoid = Navicular. 
Carpale (Tarsale), i. =Trapezium = Internal Cuneiform. 
Carpale (Tarsale), ii. =Trapezoid = Middle Cuneiform. 
Carpale (Tarsale), 11. =Os Magnum = External Cuneiform. 
Carpale (Tarsale), iv.) _ 
Carpale (Tarsale), v. J van =Cuboid. 
The pisiform is omitted from the above table, since it is now generally regarded as being a 
vestige of an additional digit placed post-axial to the little finger (digitus post-minimus). Its 
homologue in the foot is by some considered as fused with the os calcis. Similarly, on the pre- 
axial border of the hand and foot, vestiges of a suppressed digit (prepollex and prehallux) may 
