MOEPHOLOGY OF THE LIMBS. 295 



pre-axial (radial) side of the forearm is now directed forwards and somewhat laterally, whilst 

 in the hind limb the pre-axial (tibial) side of the leg is turned backwards and laterally the 

 pre-axial borders of the hand with thumb, and foot with great toe being in correspondence. ' In 

 consequence of these changes in the position of the limbs, amounting in all in the upper segments 

 to a rotation through an angle of 90, the extensor surface of the fore limb is directed backwards 

 whilst that of the hind limb is directed forwards. In order to homologise the arrangement of 

 the bones in the extended limb, it is necessary to place them so that their flexor or extensor 

 surfaces are similarly disposed. It will then be observed (see diagram) that the medial or tibial 

 side of the leg and foot (primitively pre-axial) corresponds to the lateral or radial side of the 

 forearm and hand (primitively pre-axial), whilst the fibula and lateral border of the. foot 

 homologise with the ulnar or medial border of the forearm and hand (primitively post - axial), 

 the result, as previously explained, of the torsion or twisting in opposite directions through an 

 angle of 90 of the upper segment of the limb. In accordance with this view, it will be 

 evident that in the fore limb there is nothing homologous with the patella, whilst in the hind 

 limb there is no part to represent the olecranon. 



In the axial mesoderm of each member, differentiation into cartilaginous segments begins 

 about the second month ; each of these cartilages becomes invested 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. 



The homodynamy of the carpal and tarsal elements may be tabularly expressed, and compared 

 i with the more generalised types from which they are evolved. 



Type. Hand. Foot. 



Radiale (Tibiale) = Navicular (body) = Talus. 



Intermedium =0s lunatum = Absent, or Os trigonum (?). 



Ulnare (Fibulare) = Os triquetrum = Calcaneus. 



Centrale = Absent, or fused with Navicular = Navicular, less its tuberosity. 



Carpale (Tarsale), i. = Os multangulum majus = First Cuneiform. 



Carpale (Tarsale), ii. = Os multangulum minus = Second Cuneiform. 



Carpale (Tarsale), iii. = Capitate = Third Cuneiform. 



Carpale (Tarsale), iv.^\ _ ,, = Cuboid, plus the peroneal . 



Carpale (Tarsale), v. / ~ sesamoii 



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 calcaneus. The tuberosity of the 

 navicular, formed, as has been stated, of three elements, of which the sesamoid bone in the 

 tendon of the tibialis posterior may be one, is to be regarded as the homologue of the pre-axial 

 sesamoid in the hand, which probably fuses with the navicular to form its tuberosity. The 

 peroneal sesamoid probably corresponds to the hamulus (sometimes an independent ossicle) 

 of the os hamatum. Similarly, on the pre-axial border of the hand and foot, vestiges of a 

 suppressed digit (prepollex and prehallux) may occasionally be met with. The frequent occur- 

 rence of an increase in the number of digits seems to indicate that phylogenetically the number 

 of digits was greater than at present, and included a prepollex or prehallux, and a digitus post- 

 minimus. The correspondence of the metacarpus with the metatarsus and the phalanges of the 

 fingers with those of the toes is so obvious that it is sufficient merely to mention it. 



The differences in size, form, and disposition of the skeletal elements of the hand and foot is 

 easily accounted for by a reference to the functions they subserve. 



In the hand, strength is sacrificed to mobility, thus leading to a reduction in the size of the 

 carpal elements, and a marked increase in the length of the phalanges. The freedom of move- 

 ment of the thumb, and its opposability to the other digits, greatly enhances the value of 

 the hand as a grasping organ. In the foot, where stability is the main requirement, the tarsus 

 is of much greater proportionate size, whilst the phalanges are correspondingly reduced. Since 

 the foot no longer serves as a grasping organ, the great toe is not free and opposable like the 

 thumb. 



Limb Girdles. The free limbs are linked to the axial skeleton by a chain of bones which 

 constitute their girdles. The fundamental form of these limb girdles consists each of a pair of 

 curved cartilages placed at right angles to the axis of the trunk on either side, and embedded 

 within its musculature. Each cartilage has an articular surface laterally, about the middle, for 

 the reception of the cartilage of the first segment of the free limb. In this way each pectoral 

 and pelvic cartilage is divided into an upper or dorsal half and a lower or ventral half. The 

 dorsal halves constitute the scapula and ilium of the pectoral and pelvic girdles respectively. 

 With regard to the ventral halves there is more difficulty in establishing their homologies. The 

 original condition is best displayed in the pelvic girdle ; here the ventral segment divides into 

 two branches one anterior, which represents the pubis, the other posterior, which ultimately 

 i forms the ischium. Ventrally, the extremities of these cartilages unite to enclose the obturator 

 foramen. In the pectoral girdle the disposition of the ventral cartilages is not so clear, consisting 

 primitively of an anterior branch or precoracoid, and a posterior portion or coracoid ; these, in 

 higher forms, have undergone great modifications in adaptation to the requirements of the fore 

 limbs. The posterior or coracoid element, the homologue of the ischial cartilage in the pelvic 

 girdle, is but feebly represented in man by the coracoid process and the coraco-clavicular ligament. 



