-(), STRUCTURE OF VERTEBRATES 



across the ulna and the thumb to the inside. The limb is then in 

 the position adopted in the ordinary ' press-up ' of the gymnasium. 

 The only common further development is a straightening of 

 the limb so that in the standing position all the bones, from 

 humerus to the terminal phalanges, run more or less in one line ; 

 this is seen at its best in the horse or elephant. 



The hind-limb goes through a similar series of changes, which 

 cannot be illustrated in the human body. In the mammals, 

 however, the femur rotates forward, not backwards hke the 

 humerus. The result of this is that while in both limbs the preaxial 

 digit (thumb or big toe) comes to lie on the inside (medial in 

 anatomical terminology), the preaxial border of the humerus is 

 outside (lateral in anatomical terminology) and that of the femur 

 is inside. Since also the rotation of the femur and that of the shank 

 are in the same direction, the tibia does not have to cross over 

 the fibula as the radius does over the ulna. As has been seen, the 

 forearm of man retains much freedom of movement. In its 

 primitive position, with radius and ulna parallel, it is said to be 

 supine ; when they are crossed, to bring the thumb inside, it is 

 said to be prone. 



VERTEBRA 



The segmentation of chordates is expressed chiefly in the meso- 

 derm, and is seen at its simplest in the embryo of Branchiostoma, 

 where the mesoderm develops as a series of paired pouches, 

 each containing a part of the coelom (Fig. 482). Ventrally the 

 segmentation is lost, but each somite develops dorsally into the 

 segmental muscle which is visible in the adult. In the embryos 

 of vertebrates there is a similar, though often not so clear, 

 arrangement of the somites, well seen in the second day chick. 

 Each somite (Fig. 441) early divides into a number of parts, of 

 which the chief are the myotome, which forms muscle, the nephro- 

 tome, which will be referred to below, and the sclerotome. In fishes 

 the muscles retain their segmental arrangement, but in land 

 animals, largely because of the new importance of the limbs 

 and the rearrangement of the muscles which is necessary in 

 order to work them, segmentation can only be made out in the 

 embryo . 



In all vertebrates the primary axial skeleton is the notochord, 

 which is unsegmented, but it is always replaced more or less 

 completely by the vertebral column. The development of this is 



