THE LIMBS. 



291 



The Internal Differentiation of Tissues begins at the basal part of 

 the limb and spreads towards the fingers, the terminal phalanges 

 being the last of the skeletal parts to become differentiated. The 

 mesoblast becomes condensed in the axis of the bud and forms the 

 fibrous basis of the limb bones. The skeletal basis of mesoblast 

 is continuous, but where joints are to be formed there occur 

 opener formations in the arrangement of the cells. In the 5th 

 week the fibrous basis of the bones begins to chondrify, and this 

 is soon followed by the appearance of centres of ossification in the 

 shafts of the long bones (7th week). The mesoblast between the 

 chondrified bases of the bones opens out into a cavity and forms 

 the synovial membranes of the joints. The joints are all formed 

 before the 3rd month is well begun. During the 6th to the 

 8th week the mesoblast differentiates into muscles, bones, vessels 

 and the sheaths of nerves ; the tissue left over, not included in 

 these structures, forms their sheaths, and the fasciae and con- 

 nective tissue of the limb. The processes of the nerve cells to 

 form the nerves, and of the muscle plates to form the muscles, 

 grow in very early (see Fig. 237). 



epiphysis of cresfcsupra- scapula 



<?ax. border) 



rect. fern. (B) 



acetab. (A) 

 ~pub=clav. 



long head 

 triceps (B) 



glenoid (A) 



ischium=coraeoid 



Fig. 236.— The Corresponding Points {A, B, C, and D) in the Ilium and Scapula. 



Torsion and Rotation of the Limbs.— As the limbs are 

 developed, the extensor surfaces of the knee and elbow are 

 directed upwards. If the body of an adult were placed in the prone 

 position, it would be necessary, in order to restore the limbs to 

 their embryonic position, (1) to draw them out at right angles to 



