294 



OSTEOLOGY. 



APPENDIX F. 



MORPHOLOGY OF THE LIMBS. 



Development and 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 corresponding 

 number of segmental nerves, and the circum- 

 stance 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 correspond- 

 ing to the future flexor surface of the limb, 

 the dorsal to the extensor side. At the 

 same time, the borders are directed head- 

 wards (pre-axial), and tailwards (post-axial). 

 As the limbs grow, they soon display 

 evidence of division into segments corre- 

 sponding to the hand and foot, forearm ai 

 leg, upper arm and thigh. Coincident wit! 

 this (about the second month) the cartil- 

 aginous framework of the limb is beii 

 differentiated. The disposition of the 

 cartilages furnishes a clue to their homo- 

 logics. In the fore-limb the radius ai 

 thumb lie along the pre-axial borders, anc 

 correspond to the tibia and great toe, whicl 

 are similarly disposed in the hind-limb 

 whilst the ulna and fifth finger are hom( 

 logons with the fibula and fifth toe, whk 

 are in like manner arranged in relation 

 the posterior (post-axial) border of theii 

 respective limbs. Up to this time the liml 

 are directed obliquely ventralwards. During 

 the third month a change in the position 

 of the limbs takes place, associated with 

 the assumption of the foetal position. 

 Owing to the elongation of the limbs, 

 they become necessarily bent at the elbow 

 and knee, the upper arm inclining down- 

 ward along the thoracic wall, whilst the 

 thigh is directed upwards in contact with 

 the abdominal parietes. At the same time a 

 rotation of each of these segments of the limb 

 takes place in an inverse direction, so that the 

 pre-axial border of the humerus is turned 

 laterally, whilst the pre-axial border of the 

 femur is turned medially. Assuming that 

 these borders are homologous, it results from 

 this, that the lateral condyle of the humerus 

 corresponds to the medial condyle of the 

 femur. This torsion of the limb is in part 

 'effected at the shoulder and hip joints, 

 and to some extent also in the shafts of the 

 bones. Some anatomists 'hold that this 

 rotation is not confined to the limb, but 

 involves the dorsal part of the limb girdles. 

 Others maintain that there is no evidence 

 that such takes place. In the upper limb, 

 owing to a certain amount of pronation, the 



Fio. 284. DIAGRAM TO ILLUSTRATE THE HOMOLOGIES OF 



THE BONES OF THE LlMBS. 



The two limb buds of an embryo prior to flexion and rotation. 

 The anterior or pre-axial border is coloured red; the 

 posterior or post-axial border, blue. B. After the 

 assumption of the foetal position. Flexion and rotation 

 have now taken place. The red and blue lines indicate 

 the altered position of the pre-axial and post-axial 

 borders. C. The fully developed limbs with the flexor 

 aspects directed towards the reader. The coloured 

 lines indicate the effect of the torsion of the upper 

 segment of the limb through quarters of a circle. 



